[mypyc] Add new type for expressing header dependencies

[p-sawicki]

Follow-up to #21183 to address a review comment. The external dependency of a capsule added there is really only needed to get the path of a lib-rt header that can be placed in the external header of a C extension so it doesn't make sense to use SourceDep that references a .c file. Instead add a new HeaderDep type that expresses only a dependency on a header file.

The header has to be included in the external C extension header because the exports table might reference types defined in lib-rt headers, like VecT in lib-rt/vecs/librt_vecs.h. However, the external C extension header cannot include the _api.h header added in that PR because that would leak a dependency on the associated _api.c file which defines extern objects declared in _api.h. That would result in undefined objects when a C extension that doesn't depend on a given lib-rt module (including indirectly) imports from a C extension that does.