Fix local asset manager and duplicate loaders / savers being scanned

This fixes the local asset manager so it doesn't crash with a dependency
injection error when the asset manager starts.

It also fixes an issue where multiple loaders / savers / editors of the
same type could be registered if the location of the asset manager and
the current working directory were the same, and there were other
libraries that contained asset loaders / savers / editors to register.

Maniphest Tasks: T119
Fix issue where NetworkAssetManager doesn't permit GetAll call.

Running the asset manager in networked mode requires it have the ability
to scan all assets with GetAll.  Without the [ClientCallable] attribute,
the network manager crashes with MemberAccessException.  This adds the
attribute so the asset manager can scan for all assets.

Maniphest Tasks: T119