Support subfolders in mu-plugins

[onnimonni]

Currently we are able to load plugins and themes from subfolders like

wp-content/plugins/plugin-subfolder

but somehow we don't use the same logic for mu-plugins.

I would want

wp-content/mu-plugins/plugin-subfolder

to act in similiar manner.

I have been using bedrock-autoloader.php for long time for this but I think it's finally time to support this feature without extra hacks.

For example if I use a hack like bedrock-autoloader.php the wp-cli won't list the used mu-plugins from subfolders as activated mu-plugins causing confusion for admins.

Supporting subfolders from mu-plugins helps using dependency managers like composer.

Links:
​https://github.com/roots/bedrock/blob/master/web/app/mu-plugins/bedrock-autoloader.php

[SergeyBiryukov]

Duplicate of #14359.