capture extra_rpaths?

[cyrush]

this is more of a spack issue, but I wanted to document here b/c it was pothole on my adventure to use spack envs.

I noticed that many of our packages have custom rpath logic (compiler or machine specific) baked into the package.py.

Ideally we can provide this info via extra_rpaths when we setup a compiler in our spack environment.

But, if we use extra_rpaths - I don't think this logic is captured by the CMakeCachedPackage and I am not sure how to access it in a packages.py.