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.
This disallows building an interface file that imports such module which should be allowed
since interface does not contain package symbols unless usableFromInline or inlinable.
This change limits erroring only when building a .swift file.
Resolves rdar://108633068
