Import inefficiency: interleaving and already imported detection are inverted

[Robie Basak]

I noticed that a package that has always been in sync in Ubuntu is causing the importer to start back from the beginning of Ubuntu history. This is harmless except for the waste of resources.

The reason is that launchpad_versions_published_after() against Ubuntu will never find a match. So it will require imports from the beginning of Ubuntu.

Instead we should move the _spi_found_in_relevant_head() detection to after the results from Debian and Ubuntu are interleaved. As soon as we find an import that has been performed in the interleaved output, we know that all previous imports have been completed.

This is low priority because it's only an observation that this is wasteful. I haven't had any indication that this is causing a problem anywhere. The two places it might cause an issue is in importer load and load to Launchpad to re-download the additional source packages.

Any change to the algorithm in this area needs to be done very thoughtfully to avoid corrupting importer output.