We need to generalize 'package-with-python2'

[Mark H Weaver]

The 'python-dbus' package (called 'dbus-python' upsteam) uses the
'gnu-build-system', and like most other python libraries, there will be
two variants: 'python-dbus' and 'python2-dbus'.  Unfortunately,
'package-with-python2' only works on packages that use
'python-build-system'.  This is a problem.

It wouldn't be so bad to use 'inherit' to create 'python2-dbus' from
'python-dbus', but the more serious problem is that unless we generalize
'package-with-python2', it won't work properly on any package that uses
'python-dbus' as an input.

On the other hand, I realize that 'package-with-explicit-python'
intentionally avoids traversing into packages unless they use
'python-build-system', and that this is important for efficiency.

Any thoughts on how best to solve this?

      Mark