spack uninstall no longer requires a known package. (#3915)

- Spack install would previously fail if it could not load a package for
  the thing being uninstalled.

- This reworks uninstall to handle cases where the package is no longer
  known, e.g.:
    a) the package has been renamed or is no longer in Spack
    b) the repository the package came from is no longer registered in
       repos.yaml