[mod] revision of the settings_loader

The intention of this PR is to modernize the settings_loader implementations.
The concept is old (remember, this is partly from 2014), back then we only had
one config file, meanwhile we have had a folder with config files for a very
long time.  Callers can now load a YAML configuration from this folder as
follows ::

    settings_loader.get_yaml_cfg('my-config.yml')

- BTW this is a fix of #3557.

- Further the `existing_filename_or_none` construct dates back to times when
  there was not yet a `pathlib.Path` in all Python versions we supported in the
  past.

- Typehints have been added wherever appropriate

At the same time, this patch should also be downward compatible and not
introduce a new environment variable. The localization of the folder with the
configurations is further based on:

    SEARXNG_SETTINGS_PATH (wich defaults to /etc/searxng/settings.yml)

Which means, the default config folder is `/etc/searxng/`.

ATTENTION: intended functional changes!

 If SEARXNG_SETTINGS_PATH was set and pointed to a not existing file, the
 previous implementation silently loaded the default configuration.  This
 behavior has been changed: if the file or folder does not exist, an
 EnvironmentError exception will be thrown in future.

Closes: https://github.com/searxng/searxng/issues/3557
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

searx/plugins/hostnames.py

@@ -96,7 +96,7 @@ from flask_babel import gettext
 
 from searx import settings
 from searx.plugins import logger
-from searx.settings_loader import get_yaml_file
+from searx.settings_loader import get_yaml_cfg
 
 name = gettext('Hostnames plugin')
 description = gettext('Rewrite hostnames, remove results or prioritize them based on the hostname')
@@ -118,7 +118,7 @@ def _load_regular_expressions(settings_key):
 
     # load external file with configuration
     if isinstance(setting_value, str):
-        setting_value = get_yaml_file(setting_value)
+        setting_value = get_yaml_cfg(setting_value)
 
     if isinstance(setting_value, list):
         return {re.compile(r) for r in setting_value}
@@ -163,10 +163,10 @@ def _matches_parsed_url(result, pattern):
 def on_result(_request, _search, result):
     for pattern, replacement in replacements.items():
         if _matches_parsed_url(result, pattern):
-            logger.debug(result['url'])
+            # logger.debug(result['url'])
             result[parsed] = result[parsed]._replace(netloc=pattern.sub(replacement, result[parsed].netloc))
             result['url'] = urlunparse(result[parsed])
-            logger.debug(result['url'])
+            # logger.debug(result['url'])
 
         for url_field in _url_fields:
             if not result.get(url_field):