Add support for targets with mixed language sources

[ncooke3]

Add support for targets with mixed language sources.

Motivation:

At the time of this PR, the package manager does not support targets with mixed language sources. This feature was motivated by the need for packages to contain mixed language sources for legacy and technical reasons.

Modifications:

Detailed design is explained in swiftlang/swift-evolution#1895

The diff is large but most of it comes from the suite of integration tests. I added many fixture tests to test many cases that arise when making a mixed target.

High Level Overview

1. Added a MixedTarget type to model targets with mixed language sources
   • The current approach is to effectively wrap a SwiftTarget and ClangTarget.
2. Added a MixedTargetBuildDescription type to model how MixedTarget's should build.
   • The current approach is to effectively wrap a SwiftTargetBuildDescription and ClangTargetBuildDescription.
3. Expanded upon the existing VFSOverlay type to support VFS overlay directories.
   • Used by MixedTargetBuildDescription when creating the VFS overlays needed to compile a mixed language target.
4. Expanded upon the existing ModuleMapGenerator implementation to generate module maps for mixed language targets. This involves adding a submodule to the module map to expose the generated Swift interop header.
5. Modify the PackageBuilder to created a mixed language target instead of throwing an error when mixed language sources are detected (when sufficient Swift tools version is detected).
6. Add unit tests to test planning a mixed target, mixed target module map generation, and building a mixed target.
7. Add extensive suite of integration (in FunctionalTests target) tests to test mixed target use cases.

Result:

Targets with mixed language sources can be built, tested, and depended on by other packages.

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Development Workflow

I have added info to this Google Doc for instructions on how to use this implementation to build and test mixed language targets.

[MaxDesiatov]

@swift-ci please smoke test

[neonichu]

Thanks a lot for this, @ncooke3!

The changes look good to me in general, I'll do are more in-depth review once we're closer to landing this. It would be good to unify mixed targets and the individual language targets at some point, but it seems reasonable to start this way.

[neonichu]

One bit I hadn't thought about until now is that we'd need to make the support conditional on the tools-version since we would not want authors accidentally creating packages that aren't actually backwards compatible. For existing tools-versions, we would probably want to preserve the error message about mixed sources not being supported.

[ncooke3]

In 9456663, I added a suite of integration-style tests to document and test the behavior of mixed targets. Along the way, I tweaked the implementation, and added TODOs to further refine the implementation. Currently, not all of the added tests are passing– there are some gaps that I will be filling in over the next few days.

I'm happy to break the PR up once it is ready for review.

[paulb777]

Looks great! Just some miscellaneous nits ....

[MaxDesiatov]

@swift-ci test