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.
ambiguity during lookup when there are multiple public or inlinalbe
package decls in public interfaces. This PR adds a package name to
public swiftinterface and lets typecheck look up the package name
to narrow down the scope of access to package decls from an external
module.
Resolves rdar://117699160
