Add support for swift modules with swiftinterfaces (build_library_for_distribution mode)

[vasvf]

This adds .swiftinterface files to artifacts that are needed for swift-only modules that contain @objc exposing interfaces.

Also tried to modify tests to fit this new type of file, but maybe I missed something

[polac24]

That seems to be a reasonable change that is useful for projects with enabled BUILD_LIBRARY_FOR_DISTRIBUTION. In the PR description you mentioned adding @objc public ... but as far as I am aware, generating .swiftinterface depends for swift-evolution enabled builds (In Xcode managed by BUILD_LIBRARY_FOR_DISTRIBUTION).

Owing to that, you PR implementation seems to be fine but we should have two unit tests (in change BuildArtifactCreatorTests.swift and ArtifactSwiftProductsBuilderImplTests.swift:

• Mode without swift-evolution - that is the current stat of these tests
• mode with swift-evolution: same as above + expect zipping and unzipping extra .swiftinterface

In other words, your unit testing changes should be applied in a separate functions.

[vasvf]

Separated tests in two files you specified.

I also removed unused let swiftmoduleExtensionsToInclude in ArtifactSwiftProductsBuilder.swift

I also added swiftinterface file check in swiftcTests

[polac24]

Thanks!