Add support for exposing C/C++ dependencies

[brians-neptune]

Some Python libraries (numpy is the most common example) support C/C++ code depending on them directly. To build this C/C++ code with Bazel, we need to expose the header files.

Typically C/C++ code depending on headers from Python packages is going to be imported as other Python packages, so it does not link directly to any definitions for the APIs declared in the header files it uses. cc_srcs could be added alongside cc_hdrs_globs to enable this if there's a use case in the future.