Using more specific class.
Navigation to library sources via navigation element.

It is better because it's lazy and similar to IDEA's usual behavior.
Minor. Made method @Nullable.
Got rid of tons of warnings in tests.
Moved & renamed methods.
Removed checking for navigation element, since it is outdated.

And also, it was breaking tests.
Removed subclassing from PsiClassHolderFileStub.

It was useless and wrong.
Building light classes for Kotlin library.
Minor. Inlined field.
Got rid of protected fields.
Minor. Moved method to subclass.
Using LightProjectDescriptor in navigate to library tests.

1.5x faster execution and reusable code.
Packing mock library to jar to avoid VFS magic.
Got rid of VFS magic.