Fix gtk+3.0 jenkins job

For most packages the name of the git repository is the same as that of the source package. Unfortunately gitlab doesn't allow + in git urls, which is a problem for gtk+3.0. Adjust the packaging template a bit so we can support the git package name and the source package name being different and special case gtk+3.0

Signed-off-by: Sjoerd Simons sjoerd.simons@collabora.co.uk