Provide a subcommand to remove unneeded files

[bbannier]

zkg creates a lot of files which are not cleaned up after installation, e.g., from a user listing their /var/lib/zkg:

219M	clones
72K	logs
16K	manifest.json
37M	scratch
393M	testing

I suspect many of these files are not required anymore after installation.

We should consider either always automatically cleaning up unnecessary files after package installation, or provide a subcommand to perform house-keeping on that directory (zkg vacuum?).