buildcache: Add sub-commands needed by release workflow

Adds four new sub-commands to the buildcache command:

1. save-yaml: Takes a root spec and a list of dependent spec names,
along with a directory in which to save yaml files, and writes out
the full spec.yaml for each of the dependent specs.  This only needs
to concretize the root spec once, then indexes it with the names of
the dependent specs.

2. check: Checks a spec (via either an abstract spec or via a full
spec.yaml) against remote mirror to see if it needs to be rebuilt.
Comparies full_hash stored on remote mirror with full_hash computed
locally to determine whether spec needs to be rebuilt.  Can also
generate list of specs to check against remote mirror by expanding
the set of release specs expressed in etc/spack/defaults/release.yaml.

3. get-buildcache-name: Makes it possible to attempt to read directly
the spec.yaml file on a remote or local mirror by providing the path
where the file should live based on concretizing the spec.

4. download: Downloads all buildcache files associated with a spec
on a remote mirror, including any .spack, .spec, and .cdashid files
that might exist.  Puts the files into the local path provided on
the command line, and organizes them in the same hierarchy found on
the remote mirror

This commit also refactors lib/spack/spack/util/web.py to expose
functionality allowing other modules to read data from a url.

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