Handle UnityEditor.dll being split into modules

[citizenmatt]

Unity 2020.2 is splitting UnityEditor.dll into modules, e.g. UnityEditor.CoreModule.dll. This breaks existing external annotations for UnityEditor. Also make sure that we don't look for UnityEditor.dll without a fallback to `UnityEditor.CoreModule.dll.

See comments on #19 and #143

[aybe]

It shouldn't be possible to CreateAssetMenu for Editor type unless I'm mistaken:

[citizenmatt]

Yeah, sounds like a bug. Can you create a new issue so it doesn't get lost in the UnityEditor split work, please?

[aybe]

here #1702

[citizenmatt]

See also RIDER-46544