[MLIR][python] Export CAPI headers.

* Adds source targets (not included in the full set that downstreams use by default) to bundle mlir-c/ headers into the mlir/_mlir_libs/include directory.
* Adds a minimal entry point to get include and library directories.
* Used by npcomp to export a full CAPI (which is then used by the Torch extension to link npcomp).

Reviewed By: mikeurbach

Differential Revision: https://reviews.llvm.org/D107090