[DependencyScanning] Handle testable dependencies correctly
Teach scanner to pick and choose binary modules correctly based on if it
is testable import or not. Some situations that scanner need to be
careful when testable is involved:
* When it is a regular import, it should not import binary modules that
are built with -enable-testing, it should prefer interfaces if that is
available.
* When testable import, it should only load binary module and it should
make sure the internal imports from binary modules are actually
required for testable import to work.
If a testable import only find a regular binary module, dependency
scanner currently will just preceed with such module and leave the
diagnostics to swift-frontend, because the alternative (failed to find
module) can be confusing to users.
rdar://125914165
