concurrent installation of different pkg versions

[Daniel Pocock <daniel@pocock.pro>]

With all the talk about removing jquery from source packages, one thing
that does arise is the question of how to support different jquery versions.

This is not just a JavaScript issue though.  Maybe we can have

  libjs-jquery-1.7
  libjs-jquery-1.10

and friends all installed concurrently.  Maybe it will be needed more
widely though (e.g. for some Java stuff too).  On the other hand, some
maintainers and security team naturally don't want the hassle of
supporting so many versions concurrently.  With so many upstreams now
including stuff like this, particularly in web software and the
emergence of node.js as well, maybe a generalized approach is needed.

There was even a debate about this on the backports list recently in the
context of how to support different versions of OpenStack (not installed
concurrently though, but just making perhaps the most recent 2 releases
available to users on wheezy)