Previously package decls at the defition sites were non-resilient.
Now they are resilient by default and are printed in interface as so.
Ref: rdar://118461385
Add a new flag to enable package interface loading.
Use the last value of package-name in case of dupes.
Rename PrintInterfaceContentMode as InterfaceMode.
Update diagnostics.
Test package interface loading with various scenarios.
Test duplicate package-name.
It has an extension .package.swiftinterface and contains package decls
as well as SPIs and public/inlinable decls. When a module is loaded
from interface, it now looks up the package-name in the interface
and checks if the importer is in the same package. If so, it uses
that package interface found to load the module. If not, uses the existing
logic to load modules.
Resolves rdar://104617854
