NetKAN: Search more aggressively for .version files.

Previously, we'd only use a `.version` file if it was going to be
installed.

Now, we'll still check the installed files first, but will fall back
to using a lone `.version` file if it's the only one in the archive.

Eases #269.
Merge pull request #309 from pjf/269_avc

NetKAN: Search more aggressively for .version files.