Support package documentation imports across in-package ObjC headers

DocC builds no longer fail on Swift package targets whose public Objective-C headers import another module from the same package (e.g. Firebase). Tuist now adds a narrowly scoped target dependency when public headers reference an importable module from the same package that is not already declared, and writes the canonical Modules/module.modulemap location into the build product so DocC’s Clang ExtractAPI can find dependency framework module maps.