The "mirrors" section of "spack.yaml" should support file paths that are relative to the spack.yaml.

Rationale

I have some dependencies I've "vendored" into my source control via a spack mirror for systems where those dependencies aren't accessible. I'd like the included spack environment to point to that mirror. Currently mirrors can not be to a relative path.

Description

Allow relative file paths in the mirrors section of spack.yaml