Support exposing enums from ObjC via Bridging headers

[polac24]

Part III (out of IV) of #140 fix

• an artifact .zip for a swift target, stores two ObjC headers: -Swift.h and -Swift.h.md5. Because the former could contain absolute paths, the latter file is used in a dependency fingerprint (just like .swiftmodule files) - its content is just the same as the .swiftmodule's override (a fingerprint of the target)
• Producer and consumers for each .h dependency, check if an override is available. If so, its content is used for the fingerprint
• Because -Swift.h.md5 is automatically placed next to the -Swift.h only for a .framework, a developer that wants to cover that for a static library, has to manually add an extra ditto (similar to cp) build step to move -Swift.h.md5 along -Swift.h. Required steps are documented in Readme.md. This scenario can be considered (only static library + exposing enum via a bridging header) as an edge case - very unlikely to happen.

This part unblocks rake e2e:run_standalone added in #147.

[PatrikBillgren]

Looks really good! Approved, I just noticed one minor nit in a comment

[polac24]

@PatrikBillgren, can you please reapprove after my typo fix?

[CognitiveDisson]

@polac24 Should we merge it?

[polac24]

@CognitiveDisson, yes. let me merge it.