972880 - The ISO importer now checks all ISO units before downloading

https://bugzilla.redhat.com/show_bug.cgi?id=972880

Before, it only checked the repository it was importing to for content that already existed on disk. This would cause it to redownload content, and if this was a sync from localhost it would overwrite the content already on disk.

One thing I seriously considered was making the importer use its working directory, but this would (I believe) require a good amount of refactoring and won't be very beneficial (other than being good practice).

This also fixes https://bugzilla.redhat.com/show_bug.cgi?id=965769
Should we also add an assertion that no downloads were started?