support versioned provides as implemented by dpkg

APT supported versioned provides for a long while in an attempt to get
it working with rpm. While this support is old, we can be relatively
sure that it works as versioned provides are used internally to make
Multi-Arch:foreign work.

Previous versions of apt will print a warning indicating that the
versioned provides is ignored, so that something which "Provides: foo (=
2)" doesn't provide anything.

Note that dpkg does allow only a equals-relation in the provides line
as anything else is deemed too complex. apt doesn't support anything
else either and such a support would require potentially big changes.

Closes: 758153