[refactor] migrate plugins from "module" to class SXNGPlugin

This patch brings two major changes:

- ``Result.filter_urls(..)`` to pass a filter function for URL fields
- The ``enabled_plugins:`` section in SearXNG's settings do no longer exists.

To understand plugin development compile documentation:

    $ make docs.clean docs.live

and read http://0.0.0.0:8000/dev/plugins/development.html

There is no longer a distinction between built-in and external plugin, all
plugins are registered via the settings in the ``plugins:`` section.

In SearXNG, plugins can be registered via a fully qualified class name.  A
configuration (`PluginCfg`) can be transferred to the plugin, e.g. to activate
it by default / *opt-in* or *opt-out* from user's point of view.

built-in plugins
================

The built-in plugins are all located in the namespace `searx.plugins`.

.. code:: yaml

    plugins:

      searx.plugins.calculator.SXNGPlugin:
        active: true

      searx.plugins.hash_plugin.SXNGPlugin:
        active: true

      searx.plugins.self_info.SXNGPlugin:
        active: true

      searx.plugins.tracker_url_remover.SXNGPlugin:
        active: true

      searx.plugins.unit_converter.SXNGPlugin:
        active: true

      searx.plugins.ahmia_filter.SXNGPlugin:
        active: true

      searx.plugins.hostnames.SXNGPlugin:
        active: true

      searx.plugins.oa_doi_rewrite.SXNGPlugin:
        active: false

      searx.plugins.tor_check.SXNGPlugin:
        active: false

external plugins
================

SearXNG supports *external plugins* / there is no need to install one, SearXNG
runs out of the box.

- Only show green hosted results: https://github.com/return42/tgwf-searx-plugins/

To get a developer installation in a SearXNG developer environment:

.. code:: sh

   $ git clone git@github.com:return42/tgwf-searx-plugins.git
   $ ./manage pyenv.cmd python -m \
         pip install -e tgwf-searx-plugins

To register the plugin in SearXNG add ``only_show_green_results.SXNGPlugin`` to
the ``plugins:``:

.. code:: yaml

    plugins:
      # ...
      only_show_green_results.SXNGPlugin:
        active: false

Result.filter_urls(..)
======================

The ``Result.filter_urls(..)`` can be used to filter and/or modify URL fields.
In the following example, the filter function ``my_url_filter``:

.. code:: python

   def my_url_filter(result, field_name, url_src) -> bool | str:
       if "google" in url_src:
           return False              # remove URL field from result
       if "facebook" in url_src:
           new_url = url_src.replace("facebook", "fb-dummy")
           return new_url            # return modified URL
       return True                   # leave URL in field unchanged

is applied to all URL fields in the :py:obj:`Plugin.on_result` hook:

.. code:: python

   class MyUrlFilter(Plugin):
       ...
       def on_result(self, request, search, result) -> bool:
           result.filter_urls(my_url_filter)
           return True

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

searx/plugins/oa_doi_rewrite.py

@@ -1,54 +1,90 @@
 # SPDX-License-Identifier: AGPL-3.0-or-later
 # pylint: disable=missing-module-docstring
-
 from __future__ import annotations
+import typing
+
 import re
-from urllib.parse import urlparse, parse_qsl
+from urllib.parse import parse_qsl
 
 from flask_babel import gettext
+from searx import get_setting
+from searx.plugins import Plugin, PluginInfo
+from searx.extended_types import sxng_request
 
-from searx import settings
+from ._core import log
+
+if typing.TYPE_CHECKING:
+    from searx.search import SearchWithPlugins
+    from searx.extended_types import SXNG_Request
+    from searx.result_types import Result, LegacyResult
+    from searx.plugins import PluginCfg
+
+
+ahmia_blacklist: list = []
+
+
+def filter_url_field(result: "Result|LegacyResult", field_name: str, url_src: str) -> bool | str:
+    """Returns bool ``True`` to use URL unchanged (``False`` to ignore URL).
+    If URL should be modified, the returned string is the new URL to use."""
+
+    if field_name != "url":
+        return True  # use it unchanged
+
+    doi = extract_doi(result.parsed_url)
+    if doi and len(doi) < 50:
+        for suffix in ("/", ".pdf", ".xml", "/full", "/meta", "/abstract"):
+            doi = doi.removesuffix(suffix)
+        new_url = get_doi_resolver() + doi
+        if "doi" not in result:
+            result["doi"] = doi
+        log.debug("oa_doi_rewrite: [URL field: %s] %s -> %s", field_name, url_src, new_url)
+        return new_url  # use new url
+
+    return True  # use it unchanged
+
+
+class SXNGPlugin(Plugin):
+    """Avoid paywalls by redirecting to open-access."""
+
+    id = "oa_doi_rewrite"
+
+    def __init__(self, plg_cfg: "PluginCfg") -> None:
+        super().__init__(plg_cfg)
+        self.info = PluginInfo(
+            id=self.id,
+            name=gettext("Open Access DOI rewrite"),
+            description=gettext("Avoid paywalls by redirecting to open-access versions of publications when available"),
+            preference_section="general",
+        )
+
+    def on_result(
+        self,
+        request: "SXNG_Request",
+        search: "SearchWithPlugins",
+        result: "Result",
+    ) -> bool:  # pylint: disable=unused-argument
+        if result.parsed_url:
+            result.filter_urls(filter_url_field)
+        return True
 
 
 regex = re.compile(r'10\.\d{4,9}/[^\s]+')
 
-name = gettext('Open Access DOI rewrite')
-description = gettext('Avoid paywalls by redirecting to open-access versions of publications when available')
-default_on = False
-preference_section = 'general/doi_resolver'
-
 
 def extract_doi(url):
-    match = regex.search(url.path)
-    if match:
-        return match.group(0)
+    m = regex.search(url.path)
+    if m:
+        return m.group(0)
     for _, v in parse_qsl(url.query):
-        match = regex.search(v)
-        if match:
-            return match.group(0)
+        m = regex.search(v)
+        if m:
+            return m.group(0)
     return None
 
 
-def get_doi_resolver(preferences):
-    doi_resolvers = settings['doi_resolvers']
-    selected_resolver = preferences.get_value('doi_resolver')[0]
+def get_doi_resolver() -> str:
+    doi_resolvers = get_setting("doi_resolvers")
+    selected_resolver = sxng_request.preferences.get_value('doi_resolver')[0]
     if selected_resolver not in doi_resolvers:
-        selected_resolver = settings['default_doi_resolver']
+        selected_resolver = get_setting("default_doi_resolver")
     return doi_resolvers[selected_resolver]
-
-
-def on_result(request, _search, result) -> bool:
-
-    if not result.parsed_url:
-        return True
-
-    doi = extract_doi(result['parsed_url'])
-    if doi and len(doi) < 50:
-        for suffix in ('/', '.pdf', '.xml', '/full', '/meta', '/abstract'):
-            if doi.endswith(suffix):
-                doi = doi[: -len(suffix)]
-        result['url'] = get_doi_resolver(request.preferences) + doi
-        result['parsed_url'] = urlparse(result['url'])
-        if 'doi' not in result:
-            result['doi'] = doi
-    return True