Issue Description:

libSwiftPM vends various information to clients such as IDEs, and for a ClangTarget, this include such things as the path of the `include` (public-headers) directory. It does not, however, include any information about the module map type of the ClangTarget when a custom module map isn't provided.

Instead, this is something that the BuildPlan figures out right before building, but this means that IDEs that use libSwiftPM have to replicate the heuristics for what kind of module map to generate, if any.

SwiftPM should determine this when creating the ClangTarget so it can be used consistently by both SwiftPM and clients that use libSwiftPM.