Discover plugins in directory

[rafalp]

This PR implements new plugin discovery and loading system for #1524

TODO

• Replace plugin list with plugin directory
• Install requirements
  • Install .[dev] requirements by plugins/plugin-dir/pyproject.toml file
  • Install requirements by plugins/plugin-dir/requirements.txt file
  • Install requirements by plugins/plugin-dir/requirements-dev.txt file
• Include plugins
  • Glob plugins manifests from plugins/plugin-dir/plugin_package/misago_plugin.py files
  • Include plugins in INSTALLED_APPS
  • Include plugins in Python path
• Tests
• Docs

[rafalp]

I've committed first work on this: new plugin dependency installation.

1. Check if $MISAGO_PLUGINS dir exists
2. If dir exists, glob for $MISAGO_PLUGINS/*/*/misago_plugin.py
3. For each path with file like that, install .[dev] requirements from $MISAGO_PLUGINS/*/pyproject.toml.
4. If pyproject.toml doesn't exist, try installing $MISAGO_PLUGINS/*/requirements.txt.
5. If pyproject.toml doesn't exist, try installing $MISAGO_PLUGINS/*/requirements-dev.txt.

pyproject.toml and requirements* files are exclusive, but if pyproject.toml doesn't exist, installation from both requirements.txt and requirements-dev.txt is attempted.

In prod (eg. misago-docker) I will skip dev installation.

Its possible to have multiple misago_plugin.py in single plugin and this will result in same requirements installed multiple times, but that won't be officially supported. I just feel its such an edge case that's not worth worrying about.

Next todo: discover plugins from Python, add them to Python path, INSTALLED_APPS and gather their manifests.

[coveralls]

coverage: 98.599% (+0.04%) from 98.56%
when pulling c8580d8 on fix-1524-plugin-system-redo
into 2692e2a on main.