Issue Description:

The -emit-pcm flag was added to swiftc by apple/swift#28107 so build systems can emit explicit precompiled modules of Clang module maps that are compatible with Swift's ClangImporter.

As part of its build plan, SwiftPM could emit a PCM for any C/Obj-C modules in a dependency graph and then pass those to Swift compilations that depend on them, instead of passing the module map or relying on it being found via search paths, both of which use the implicit module cache.

(While it would be nice to go a step further to 100% explicit modules and turn on -fno-implicit-modules, that would require compiling explicit modules for all the system framework/header modules as well and connecting their inter-dependencies, and would also require users to explicitly declare their dependencies on them, which is probably infeasible for SwiftPM.)