Fixes #1853.

The gist of the problem is that

self._constraints_map = {key_from_ireq(ireq): ireq for ireq in constraints}

only kept the last ireq for each key, rather than combining them (and thus their sources).

There are two changes here:

• self._constraints_map is no longer constructed directly from constraints; rather, the items in constraints are first grouped by key into sets, then those sets are "collapsed" via combine_install_requirements() into single InstallRequirements.
• pinned_ireq._source_ireqs is no longer always a singleton list of the value from self._constraints_map; rather, if the constraint has _source_ireqs, then that list is used as pinned_ireq._source_ireqs. This effectively "unwraps" _source_ireqs (once) if possible.
  • A more sophisticated solution might be to just have writer.py recurse on _source_ireqs when constructing required_by, but that touches more code paths than are necessary to fix this, so I opted for this solution instead. Such recursion would enable combined InstallRequirements to be combined further without issue, but that doesn't seem to be a requirement right now.

The new test_many_inputs_includes_all_annotations is parametrized for 2, 3, and 10 inputs; the 2 case was not broken but should still be tested.

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• Provided the tests for the changes.
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