[ScanDependency] Do not use public/private swiftinterface in the same package
When scanning finds a dependency in the same package, do not load
public/private swiftinterface since they do not have the package level
decl to compile the current module. Always prefer package module (if
enabled), or use binary module, unless it is building a public/private
swiftinterface file in which case the interface file is preferred.
This also does some clean up to sync up the code path between implicit
and explicit module finding path.
rdar://122356964