Minor. Renamed method.
Allowed "rewriting" unresolvedCache elements.

This is used to fix tests. The fix is pretty dirty, but related code is going to be thrown away anyway.
Fixed resolving references to packages in plugin.
Renamed method and got rid of some usages.
~Big refactoring. Migrating to package views and fragments.
~Renamed method and got rid of some usages.
Removed DescriptorFinder.findPackage.
Extracted method for finding exactly one fragment.
~Big refactoring. Migrating to package views and fragments.
Locking scopes of MutablePackageFragmentDescriptors.
Simplified method.
Restored test code for deserialized scopes.
~Locking scopes of MutablePackageFragmentDescriptors.
~Extracted method for finding exactly one fragment.
~Big refactoring. Migrating to package views and fragments.
Minor. Nullability annotations.
[WIP] fragment providers added on higher level