Detect plugins recursively

[Shatur]

Recursion doesn't go deeper after finding plugin.cfg (to avoid including subplugins). Also plugin_addons now contains full paths with the configuration file (it looks even simpler and more efficient).

I do not know any plugins for 4.0, so I tested this changes on 3.2 and it works as expected.

Closes godotengine/godot-proposals#1623.
References godotengine/godot-proposals#554.

[Calinou]

This feature looks good to me (and will be needed in some form for the new asset library in the future), although I haven't tested this PR.

[akien-mga]

I also think it's a good idea, and the code changes look fine (I haven't tested though). Let's merge and see how it goes in the master branch before considering merging the 3.2 version too.

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉

[Jummit]

Let's merge and see how it goes in the master branch before considering merging the 3.2 version too.

I would like to organize some plugins using folders, so any news about backporting?
This is also step one of godotengine/godot-proposals#2091

[Shatur]

@Jummit, I already backported it in #45316.

[Jummit]

@Jummit, I already backported it in #45316.

Oh, sorry. I should've seen the link to that issue in the history.