package: Avoid processing duplicated package release multiple times

It might exist cases where multiple versions of a package target the same
release object.

For instance a rpm package has one specific version for each distribution
release but they can target the same intrinsic version and source package
contents are exactly the same.

So avoid downloading and processing a package version if the corresponding
extid has already been encountered during the current loading by maintaining
a mapping between extids and release swhids.