debi: Don't use apt install ./*.changes for --with-depends --upgrade (#932963)

(This branch currently includes and requires !141 (merged), although it would be possible to cherry-pick just the fix.)

The semantics of --with-depends --upgrade have historically been that we will upgrade existing packages mentioned in the given changes file, and we will install their dependencies if necessary, but we will not install new packages mentioned in the changes file.

For example, suppose we have installed the development tool piuparts, but not the special-purpose package piuparts-master (which is only intended to be installed on piuparts master servers). If we are using debi to test a new version of piuparts that adds a dependency on uidmap, then debi --upgrade --with-depends ../piuparts.changes should install the new piuparts, and install its dependency uidmap, but should not install piuparts-master.

apt --only-upgrade is not suitable here because it would prevent us from installing uidmap; apt without --only-upgrade is also not suitable because it would install piuparts-master. Fall back to using debpkg instead.

Closes: #932963