treat .ddeb files like .deb, especially for dpkg

Ubuntu uses *.ddeb files for their debug packages, but the interface we
are using since f495992428a396e0f98886c9a761a804aa161c68 to talk to dpkg
isn't supporting *.ddeb files. This used to work previously as apt itself
isn't caring about the filenames at all and if they are explicitly
mentioned dpkg will accept all, too.

It might or might not be a good idea to patch dpkg, too, but regardless
of it happening, we don't want to couple us to closely to dpkg for this
minor feature but testing for this at runtime as it would delay shipping
the fix for the too long commandlines further.

It is also questionable if it is really a good idea to allow any file
extension to be used here (like .foobar in the testcase), but we used to
and we tend to avoid breaking existing usecases if we can help it.

As a bonus, this also allows the installation of ddeb files directly
from the commandline as you can with deb files already. We continue to
ignore udeb through as the user-mistake to useful ratio is too high.

LP: #1616909

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