Generate command lists automatically

Instead of hard-coding the list of stg subcommands everywhere,
generate them automatically. That way, they never get outdated (like
the list in stg.txt).

In order to not make StGit slower, we cache the list of commands; run
"make build" to update that cache (other make targets, such as "all"
and "test", imply "build"). If the cache doesn't exist, the code falls
back to importing all the command modules, which adds quite a bit of
overhead to each stg command (about 100 ms on my laptop).

Signed-off-by: Karl Hasselström <kha@treskal.com>