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/testdata/yt_dlp_plugins/extractor/_ignore.py

@@ -0,0 +1,5 @@
+from yt_dlp.extractor.common import InfoExtractor
+
+
+class IgnorePluginIE(InfoExtractor):
+    pass

test/testdata/yt_dlp_plugins/extractor/ignore.py

@@ -0,0 +1,12 @@
+from yt_dlp.extractor.common import InfoExtractor
+
+
+class IgnoreNotInAllPluginIE(InfoExtractor):
+    pass
+
+
+class InAllPluginIE(InfoExtractor):
+    pass
+
+
+__all__ = ['InAllPluginIE']

test/testdata/yt_dlp_plugins/extractor/normal.py

@@ -0,0 +1,9 @@
+from yt_dlp.extractor.common import InfoExtractor
+
+
+class NormalPluginIE(InfoExtractor):
+    pass
+
+
+class _IgnoreUnderscorePluginIE(InfoExtractor):
+    pass

test/testdata/yt_dlp_plugins/postprocessor/normal.py

@@ -0,0 +1,5 @@
+from yt_dlp.postprocessor.common import PostProcessor
+
+
+class NormalPluginPP(PostProcessor):
+    pass

test/testdata/zipped_plugins/yt_dlp_plugins/extractor/zipped.py

@@ -0,0 +1,5 @@
+from yt_dlp.extractor.common import InfoExtractor
+
+
+class ZippedPluginIE(InfoExtractor):
+    pass

test/testdata/zipped_plugins/yt_dlp_plugins/postprocessor/zipped.py

@@ -0,0 +1,5 @@
+from yt_dlp.postprocessor.common import PostProcessor
+
+
+class ZippedPluginPP(PostProcessor):
+    pass