Add a uniqueness constraint for source packages

[daviddavis]

Unlike other content types, there's no uniqueness constraint for SourcePackages. There's some code that does check for other source packages by source/version but since this is enforced in the code and not the db, it's possible to end up with duplicate source packages though if the package is uploaded a second time before it's saved to the db.

For a unique index, one option might be to use source/version fields which the code is using now. Another option is to use the relative_path and sha256 of the dsc file which would be somewhat consistent with other Package types (see the discussion here).