[RM-12171] Make pkg --install and --remove mutually exclusive

[RM-12171] Make pkg --install and --remove mutually exclusive

The code could not handle both being present, so make sure that we
don't allow that on the CLI.

Add unit tests for pkg.

Remove nargs=?. Using '?' doesn't make any sense unless a default
or const flag is also provided, which they aren't.  We want to make
sure there is one string provided, which is the default behavior.

Fixes: #12171
Signed-off-by: Travis Rhoden <trhoden@redhat.com>