Add inspection for targeting the same class multiple times in a mixin

[O7410]

Currently it's a weak warning

[RedNesto]

This is looking pretty good, there's a few things to note:

• this does not cover cases where a class literal and string target both reference the same class
• it would be nice to have some tests for this inspection, you can use InvalidInjectorMethodSignatureInspectionTest as a reference.
• you should be fine getting PsiClass.mixinTargets once and pass it around, instead of hitting the cache again and again

[O7410]

Take a look at the latest commit