Add FixFeature2 background worker job

[Turbo87]

This job "fixes" the features2 entries by moving all features into feature2 if feature2 already has content. This should avoid a mismatch of features pointing to features2 for older cargo versions.

Related:

• features2 in index broke cargo backward compatibility #6135
• publish: Move all features to feature2 if new syntax is used #6168

[ehuss]

It looks like this would affect 2049 files in the index, which is an uncomfortably large number. It would be good to get some data on how long that will affect the index update, particularly with a cached copy that hasn't been updated in a while (like sometime between Jan 13 and Feb 1 to be after the normalization commit).

[bors]

☔ The latest upstream changes (presumably #5066) made this pull request unmergeable. Please resolve the merge conflicts.

[Turbo87]

#9551 has fixed the original issue, but our background worker code has evolved enough to not be useful to try to rebase this PR.

we'll keep running with the new feature splitting code for a while and may perform a batch update at a later point in time to reduce the number of git deltas needed for the update.