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
Add support for cross import modules by ingesting swiftoverlay files for
the cross import into CAS file system.
The long-term better fix will be just passing the cross import
information from scanner to swift-frontend so frontend doesn't need to
read overlay files again to figure out the cross import module.
rdar://123839248
Always prefer binary module when using @testable imports because the
swiftmodule rebuilt from interface cannot be imported as testable.
rdar://123120159
'ModuleDependencyScanner' maintains a Thread Pool along with a pool of workers
which are capable of executing a filesystem lookup of a named module dependency.
When resolving imports of a given Swift module, each import's resolution
operation can be issued asunchronously.
