Spack can be extended with external commands (#8612)

This provides a mechanism to implement a new Spack command in a
separate directory, and with a small configuration change point Spack
to the new command.

To register the command, the directory must be added to the
"extensions" section of config.yaml. The command directory name must
have the prefix "spack-", and have the following layout:

  spack-X/
    pytest.ini #optional, for testing
    X/
	  cmd/
	    name-of-command1.py
	    name-of-command2.py
	    ...
    tests/ #optional
      conftest.py
	  test_name-of-command1.py
    templates/ #optional jinja templates, if needed

And in config.yaml:

  config:
    extensions:
      - /path/to/spack-X

If the extension includes tests, you can run them via spack by adding
the --extension option, like "spack test --extension=X"