Try to play nice with OpenPKG (rpm5.org) packages (rhbz#478907)

Try to play nice with OpenPKG (rpm5.org) packages (rhbz#478907)
- rpm5.org writes populates lead with partially bogus data, but
  does have explicit RPMTAG_SOURCEPACKAGE for srpms, dont think of them
  as binaries
- switches around the lead check in 50a4ed783c300a92948f7933a3c726d4bd2d643d
  to look at source type explicitly and retrofit RPMTAG_SOURCEPACKAGE
  for srpms early, this lets binaries without RPMTAG_SOURCERPM
  (which is documented as "information only") and with explicit
  RPMTAG_SOURCEPACKAGE be identified correctly