Patch Manager does not honor apt priority

This is specifically about backport packages and how mindlessly installing backports gets into a dependency hell, along with broken functionality.

By default Debian sets backports to priority 100, whereas regular repos are at 500. So backport packages are not candidates for install in apt. Yet SSM patch manager will gleefully install these backports.

The only workaround is to add a wildcard reject for anything that's got `~bpoX+Y`, so that's multiple entries for Debian9 and 10 (11 when?) and +1,2,3,etc... Haven't checked ubuntu yet.

Start by reproducing the issue with Debian backports enabled and compare the repository priorities with the packages selected by SSM Patch Manager. Trace the patch-selection path in the Go agent; done means backport packages are not selected when apt gives them lower priority, without requiring wildcard rejects.

Written by the indexing model from the issue text.