Complete RPM files instead of pacakges if there is either
1. a slash in the token, which precludes package names
2. no matching package
To enable 2, pass the commandline token to the dnf query, instead of
an undefined variable. This allows SQL injection; not sure if we care.
We could always complete RPM files but maybe that's too noisy.
Also, isn't that what the "rpm" command is for?
Closes#7928