Create depends/blocks/see-also links to gl issues, not bz bugs.

This splits the issue creation process up into a number of steps, so that when linking an issue's related bugs, it can refer to already-migrated issues rather than the old bugs instead. There's more than 2 steps since we don't want to notify watcher after updating the issue with the imported dependencies.

I broke closing the bug in bgo out as another pass as well since it can be completely skipped where there are no creds, and updated debug logging for each pass to be consistent with other phases.

Likewise, description template generation has been split up into two steps, one for the main description and one for related issues.

Tests have been added for each of the parts of the importation process and am now looking for a review, but it still needs testing against an actual instance with actual data, which I can't do since I don't have needed permissions.

Fixes #28 (closed)