core: add the short description under the source package name

It uses the description of the binary package of same name,
otherwise it just uses the first description it finds.

[hertzog@debian.org:
* Drop at test that was redundant with
  PackageViewTest.test_pseudo_package_page()
* Add a new test in SourcePackageNameTest to ensure we return the
  correct description with multiple binary packages.
* Drop the similar test in PackageViewTest.
]

Signed-off-by: Raphaël Hertzog <hertzog@debian.org>