buildpackages: honour install priorities tags, branch, sha1

buildpackages: honour install priorities tags, branch, sha1

The install config may have contradicting tag/branch and sha1.  When
suite.py prepares the jobs, it always overrides the sha1 with whatever
default is provided on the command line with --distro and what is found
in the gitbuilder. If it turns out that the tag or the branch in the
install config task is about another sha1, it will override anyway.

Instead of obtaining the tag, branch and sha1 directly from the
packaging.GitbuilderProject object, compute them from the returned
uri_reference data member. The uri_reference is used by the install task
to fetch packages in the gitbuilders and this is what buildpackages
needs to build.

Signed-off-by: Loic Dachary <loic@dachary.org>
(cherry picked from commit c103579a55cff3bd90e2d8d7b8df03bf29560961)