parse arch-qualified Provides correctly

The underlying problem is that libapt-pkg does not correctly parse these
provides. Internally, it creates a version named "baz:i386" with
architecture amd64. Of course, such a package name is invalid and thus
this version is completely inaccessible. Thus, this bug should not cause
apt to accept a broken situation as valid. Nevertheless, it prevents
using architecture qualified depends.

Closes: 777071