Most SDKs use only swiftinterfaces under usr/lib/swift. Let's make sure
we standardize this behavior and use only swiftinterface when they are
present, even if there are also binary swiftmodule files available.
Apply the same logic to SubFrameworks as well while we're at it.
rdar://145316821
In https://github.com/apple/swift/pull/42486 new behavior was introduced to ignore adjacent .swiftmodule files in the SDK. This behavior has caught a few people off guard so it seems like there should be diagnostics clarifying why a rebuild is occurring in this scenario.
Resolves rdar://105477473
Always use the swiftinterfaces of modules under the public Frameworks
folder in the SDK. Distributed swiftmodules are unreliable, they should
only be used as a local cache. Let's avoid them when possible.
rdar://92037292
