Enhance plugin loader for Must-Use plugins directory

[JoryHogeveen]

More info on mu-plugins: https://codex.wordpress.org/Must_Use_Plugins

Make installing as a mu-plugin as easy as possible:

Move the view-admin-as.php file into the root of your mu-plugins directory, not in the view-admin-as subdirectory.
(This is a limitation of WordPress and probably won't change soon.)

Example:
All files dir: /wp-content/mu-plugins/view-admin-as/...
Main file dir: /wp-content/mu-plugins/view-admin-as.php