models: index Ctag.version_id
models: index Ctag.version_id

rationale: it makes much faster verifying if the ctags of a given package have
already been inserted

nonas/debian-qa,zacchiro/debsources,nonas/debian-qa,zacchiro/debsources,matthieucan/debsources,Debian/debsources,oorestisime/debsources,matthieucan/debsources,vivekanand1101/debsources,devoxel/debsources,matthieucan/debsources,Debian/debsources,matthieucan/debsources,oorestisime/debsources,Debian/debsources,matthieucan/debsources,nonas/debian-qa,clemux/debsources,clemux/debsources,vivekanand1101/debsources,sophiejjj/debsources,devoxel/debsources,vivekanand1101/debsources,clemux/debsources,vivekanand1101/debsources,Debian/debsources,clemux/debsources,clemux/debsources,zacchiro/debsources,zacchiro/debsources,sophiejjj/debsources,vivekanand1101/debsources,devoxel/debsources,sophiejjj/debsources,devoxel/debsources,sophiejjj/debsources,oorestisime/debsources,oorestisime/debsources,devoxel/debsources,zacchiro/debsources,sophiejjj/debsources,oorestisime/debsources,Debian/debsources,nonas/debian-qa