rdar://154310191 Enable compilation caching for Swift packages

[jansvoboda11]

This PR enables compilation caching for Swift package targets whenever normal targets have it enabled.

I originally wanted to hook into the SuperimposedProperties machinery and propagate it from normal targets similar to the setting for mergeable libraries. However, some aspects of TargetDependencyResolver made that very hard:

• treating package targets differently from normal targets (see DiscoveredTargetInfo.immediateDependencies and DiscoveredTargetInfo.packageProductDependencies),
• not propagating superimposed properties transitively,
• only visiting nodes in the build graph once (even when the specializations differ).

[jansvoboda11]

@swift-ci test

[jansvoboda11]

The build log says the only test failure that's not known is coming from FileHandleTests.asyncReadFileDescriptor(). Let's try again.

@swift-ci test macOS platform