The compiler reports public imports of non-resilient modules from a
resilient module. Make sure that when imports default to internal, which
also implies Swift 6, this is treated as an error and the fixit simply
deletes the `public` keyword.
We keep using the AccessLevelOnImport flag only to report these as
errors in Swift 5 for early adopters.
Importing a non-library-evolution enabled module from a
library-evolution enabled module will lead to generating a
swiftinterface that can't be rebuilt by a different compiler.
Make it a hard error when using access level imports, and keep it as a
warning for @_implementationOnly imports.
