Wrap get_distribution_version() in the hostname module

We wrap get_distribution_version() with a new function,
_get_distribution_version(), that returns `0` when the result is a string or
`None`.

This accounts for the case when get_distribution_version() returns a string,
and we try to compare it to a float. We do this in the hostname module instead
of the module snippets because other modules may want the real string
version.module snippets because other modules may want the real string version.