init: make spack.cmd.all_commands lazy

- `spack.cmd.all_commands` does a directory listing on
  `lib/spack/spack/cmd`, regardless of whether it is needed

- make this lazy so that the directory listing won't happen unless it's
  necessary.

lib/spack/docs/conf.py

@@ -77,7 +77,8 @@
 # Find all the `cmd-spack-*` references and add them to a command index
 #
 import spack
-command_names = spack.cmd.all_commands
+import spack.cmd
+command_names = spack.cmd.all_commands()
 documented_commands = set()
 for filename in glob('*rst'):
     with open(filename) as f: