sourcepackagerelease.sourcepackage() is dangerous

Celso says:

"Sampledata is sort of inconsistent having a SPR uploaded to hoary
(mozilla-firefox_0.9) and also published in warty, this is very
counter intuitive, considering warty was released before hoary was
opened.

"However, the fact it is *very* broken is actually good, because
DistroSeriesSourcePackageRelease.sourcepackage, which is in fact
SourcePackageRelease.sourcepackage (DSSPR decorates SPR) is not what
you want to use as 'parent'.

"The SPR.sourcepackage property uses the SPR.upload_distroseries to
build a SourcePackage object and it won't be as useful as it looks
once we start supporting derivation. Let's say a source package
version uploaded to debian gets published in ubuntu (debian-sync,
which we intend to do very early in 3.0), the navigation would
mistakenly jump from one distribution to another.

"Summing up, SPR.sourcepackage is dangerous and should be renamed (or
removed)..."