* ghostscript: add version 9.21 and handle new URL format
* ghostscript: add url of latest version to fix `spack versions`
* ghostscript: use github for all versions our package provides (>= 9.18)
Package repositories now look like this:
top-level-dir/
repo.yaml
packages/
libelf/
package.py
mpich/
package.py
...
This leaves room at the top level for additional metadata, source,
per-repo configs, indexes, etc., and it makes it easy to see that
something is a spack repo (just look for repo.yaml and packages).
