support "install ./foo.changes"

support "install ./foo.changes"

We support installing ./foo.deb (and ./foo.dsc for source) for a while
now, but it can be a bit clunky to work with those directly if you e.g.
build packages locally in a 'central' build-area.

The changes files also include hashsums and can be signed, so this can
also be considered an enhancement in terms of security as a user "just"
has to verify the signature on the changes file then rather than
checking all deb files individually in these manual installation
procedures.