Most notably, this renames/moves a few important identifiers:
//:skylark_library.bzl -> //:bzl_library.bzl
skylark_library -> bzl_library
SkylarkLibraryInfo -> StarlarkLibraryInfo
As more things are added, lib.bzl is an anti-pattern as the cost of loading
it actually just keeps increasing and most things will never use everything
out of it. The pattern the should be used is to directly import the modules
one uses.