Improve plugin architecture (#5553)

to make plugins easier to develop and use:
* Plugins are now loaded as namespace packages.
* Plugins can be loaded in any distribution of yt-dlp (binary, pip, source, etc.).
* Plugin packages can be installed and managed via pip, or dropped into any of the documented locations.
* Users do not need to edit any code files to install plugins.
* Backwards-compatible with previous plugin architecture.

As a side-effect, yt-dlp will now search in a few more locations for config files.

Closes https://github.com/yt-dlp/yt-dlp/issues/1389

Authored by: flashdagger, coletdjnz, pukkandan, Grub4K
Co-authored-by: Marcel <flashdagger@googlemail.com>
Co-authored-by: pukkandan <pukkandan.ytdlp@gmail.com>
Co-authored-by: Simon Sawicki <accounts@grub4k.xyz>

test/test_plugins.py

@@ -0,0 +1,73 @@
+import importlib
+import os
+import shutil
+import sys
+import unittest
+from pathlib import Path
+
+sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
+TEST_DATA_DIR = Path(os.path.dirname(os.path.abspath(__file__)), 'testdata')
+sys.path.append(str(TEST_DATA_DIR))
+importlib.invalidate_caches()
+
+from yt_dlp.plugins import PACKAGE_NAME, directories, load_plugins
+
+
+class TestPlugins(unittest.TestCase):
+
+    TEST_PLUGIN_DIR = TEST_DATA_DIR / PACKAGE_NAME
+
+    def test_directories_containing_plugins(self):
+        self.assertIn(self.TEST_PLUGIN_DIR, map(Path, directories()))
+
+    def test_extractor_classes(self):
+        for module_name in tuple(sys.modules):
+            if module_name.startswith(f'{PACKAGE_NAME}.extractor'):
+                del sys.modules[module_name]
+        plugins_ie = load_plugins('extractor', 'IE')
+
+        self.assertIn(f'{PACKAGE_NAME}.extractor.normal', sys.modules.keys())
+        self.assertIn('NormalPluginIE', plugins_ie.keys())
+
+        # don't load modules with underscore prefix
+        self.assertFalse(
+            f'{PACKAGE_NAME}.extractor._ignore' in sys.modules.keys(),
+            'loaded module beginning with underscore')
+        self.assertNotIn('IgnorePluginIE', plugins_ie.keys())
+
+        # Don't load extractors with underscore prefix
+        self.assertNotIn('_IgnoreUnderscorePluginIE', plugins_ie.keys())
+
+        # Don't load extractors not specified in __all__ (if supplied)
+        self.assertNotIn('IgnoreNotInAllPluginIE', plugins_ie.keys())
+        self.assertIn('InAllPluginIE', plugins_ie.keys())
+
+    def test_postprocessor_classes(self):
+        plugins_pp = load_plugins('postprocessor', 'PP')
+        self.assertIn('NormalPluginPP', plugins_pp.keys())
+
+    def test_importing_zipped_module(self):
+        zip_path = TEST_DATA_DIR / 'zipped_plugins.zip'
+        shutil.make_archive(str(zip_path)[:-4], 'zip', str(zip_path)[:-4])
+        sys.path.append(str(zip_path))  # add zip to search paths
+        importlib.invalidate_caches()  # reset the import caches
+
+        try:
+            for plugin_type in ('extractor', 'postprocessor'):
+                package = importlib.import_module(f'{PACKAGE_NAME}.{plugin_type}')
+                self.assertIn(zip_path / PACKAGE_NAME / plugin_type, map(Path, package.__path__))
+
+            plugins_ie = load_plugins('extractor', 'IE')
+            self.assertIn('ZippedPluginIE', plugins_ie.keys())
+
+            plugins_pp = load_plugins('postprocessor', 'PP')
+            self.assertIn('ZippedPluginPP', plugins_pp.keys())
+
+        finally:
+            sys.path.remove(str(zip_path))
+            os.remove(zip_path)
+            importlib.invalidate_caches()  # reset the import caches
+
+
+if __name__ == '__main__':
+    unittest.main()