pkg2 wip: determine fixed packages by reachability from METADATA.

The idea here is that once a package commit is included in a version
that makes it into METADATA, you no longer need to stay on version
since it's been included upstream. The workflow here is that someone
makes a fix locally, submits it for upstream inclusion, meanwhile
the package is off-branch but with local changes, therefore fixed.
Once the fix gets included upstream and published to a version in
METADATA, it's safe to start managing the package again, so it is no
longer fixed.

In far less complicated cases, this is effective for the situation
where a package was on an off-branch commit that is no longer a
known version in METADATA but was an ancestor of one, such as when
0.0.0 is overwritten or when an older published version is "retired"
and no longer considered a candidate for installation.