Simplify and correctify calculation of the InnerClass attribute

[WojciechMazur]

This PR is a part of porting the backend improvements introduced in Scala 2 #14912 and corresponds to scala/scala#4807
Original PR description:

The InnerClass attribute needs to contain an entry for every nested
class that is defined or referenced in a class. Details are in a
doc comment in BTypes.scala.

Instead of collecting ClassBTypes of nested classes into a hash map
during code generation, traverse the class before writing it out to
disk. The previous approach was incorrect as soon as the generated
bytecode was modified by the optimzier (DCE, inlining).

Fixes https://github.com/scala/scala-dev/issues/21.

Similarly to the original repo CoreBTypes naming convention has been unified and cleaned up from not used references.
The original collection of nested classes signatures was replaced with extended version of NestedClassesCollector present in the current Scala 2 main branch https://github.com/scala/scala/blob/724be0e9425b9ad07c244d25efdad695d75abbcf/src/compiler/scala/tools/nsc/backend/jvm/analysis/BackendUtils.scala#L790
Generation of ClassBType for mirrors has been added by porting implementation from Scala 2 and definitions of MethodHandles has been updated to make sure that given ClassBTypes are loaded and reachable. This is needed since in Scala 3 we only use ClassBTypes created from Symbols, whereas in Scala 2 their instances could have been also loaded from Classfiles.