Remove triggered fixes from iterations.

[nimakarimipour]

This PR is built upon #112. #112 is landed.

Following #112 errors contain the set of fixes which can resolve the reported errors. Currently, after each fix examination, the set of triggered fixes and errors are computed per region. Since #112 updates error instances to store the resolving fixes, the set of triggered fixes (which resolves the triggered errors) is redundant. This PR removes this structure.

Changes in this PR:

• Removes set of triggered fixes from Nodes in ConflictGraph.
• Constructs fix trees based on the containing resolving fixes of triggered errors.
• Adds set of triggered fixes from downstream to nodes for clear separation between fixes from downstream and fixes from target.

[nimakarimipour]

@msridhar @lazaroclapp Thank you very much for your comments. This PR is ready for another round of review.

[nimakarimipour]

I have probably lost enough context on this PR that I might need to do another pass from the beginning, but for now, just a couple quick comments on the existing relevant discussion threads and changes since last review :)

Also, tests are failing... incomplete rebase / rebase issues? Or is it just that this one is blocked on a new NullAway version either way?

@lazaroclapp CI is blocked by the new NullAway release which includes #704

[nimakarimipour]

@msridhar @lazaroclapp Thank you for the review, this is ready for another round.