Merge lp://qastaging/~edwin-grubbs/launchpad/bug-600876-ubuntupkg-integrity-error into lp://qastaging/launchpad

Summary
-------

thunderbird-locales has packaging links for:
  Maverick -> Mozilla Thunderbird 3.1 series
  Lucid -> Mozilla Thunderbird 3.0 series

https://edge.launchpad.net/ubuntu/+source/thunderbird-locales

If you go to
    https://edge.launchpad.net/thunderbird/3.1/+ubuntupkg
and try to create packaging for 'thunderbird-locales' and
the Lucid distroseries, the view class validate() stops
checking as soon as it finds out that the sourcepackagename
is the same as the existing package on the current distroseries,
which is maverick. The action handler tries to create the packaging
link, since it is only meant to ignore the links on the same
productseries.

Implementation details
----------------------

lib/lp/registry/browser/productseries.py
lib/lp/registry/browser/tests/test_packaging.py

Tests
-----

./bin/test -vv -t test_packaging

Demo and Q/A
------------

On Dev:
* Open https://launchpad.dev/netapplet/+packages
  * Remove the package for warty, since it is on the trunk distroseries.
* Open https://launchpad.dev/netapplet/+addseries
  * Create the series.
  * Click on "Link to Ubuntu package" on the new productseries page.
  * Link it to netapplet/Warty.
* Open https://launchpad.dev/netapplet/trunk/+ubuntupkg
  * Try to link it to netapplet/Warty.
  * You should get this error message instead of an oops:
    "The netapplet package in Warty is already linked to another series."

On Edge:
* https://edge.launchpad.net/thunderbird/3.1/+ubuntupkg
  * The sourcepackagename field should say "thunderbird-locales".
  * Select Lucid from the Series drop-down.
  * Click "Update".
  * You should get an error message instead of an oops.