Convert plugin searx.plugin.limiter to normal code

The limiter plugin is not a plugin:
* the user can't enable or disable the plugin and there is no point to allow that
* the limiter does use any of the features provides by the plugin framework

This commit convert the limiter plugin into normal code

searx/botdetection/__init__.py

@@ -42,3 +42,6 @@ X-Forwarded-For
 from ._helpers import dump_request
 from ._helpers import get_real_ip
 from ._helpers import too_many_requests
+from .install import initialize, is_installed
+
+__all__ = ['dump_request', 'get_real_ip', 'too_many_requests', 'initialize', 'is_installed']

searx/botdetection/install.py

@@ -0,0 +1,43 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+# lint: pylint
+# pyright: basic
+"""see :ref:`limiter src`"""
+
+import sys
+import flask
+
+from searx import redisdb, logger
+from searx.botdetection import limiter
+
+# the configuration are limiter.toml and "limiter" in settings.yml
+# so, for coherency, the logger is "limiter" even if the module name "searx.botdetection"
+logger = logger.getChild('limiter')
+
+
+_INSTALLED = False
+
+
+def pre_request():
+    """See :ref:`flask.Flask.before_request`"""
+    return limiter.filter_request(flask.request)
+
+
+def is_installed():
+    return _INSTALLED
+
+
+def initialize(app: flask.Flask, settings):
+    """Instal the botlimiter aka limiter"""
+    global _INSTALLED  # pylint: disable=global-statement
+    if not settings['server']['limiter'] and not settings['server']['public_instance']:
+        return
+    if not redisdb.client():
+        logger.error(
+            "The limiter requires Redis, please consult the documentation: "
+            + "https://docs.searxng.org/admin/searx.botdetection.html#limiter"
+        )
+        if settings['server']['public_instance']:
+            sys.exit(1)
+        return
+    app.before_request(pre_request)
+    _INSTALLED = True