Bug#950994: Discourage package in contrib to install the real program via another package manager

[Sam Hartman <hartmans@debian.org>]

I'll note there's another case where this could be valuable.
If there is an installer in contrib, you can express dependencies on the
package being available in Debian.
Depending on how that installer works, you may not be able to express
dependencies on the installed version in Debian.

I think that unless there is a reason for such an installer, they should
be discouraged.
But I can see good reasons.
I'm not really sure that policy is the best place for this sort of
discouragement though.