Support partitioning by environment field for lint/fmt/fix/etc

[chrisjrn]

Currently, the partitioning behavior for lint/fmt/fix involves a "planning" step, where each backend chooses how to partition its work.

We're trying to make environment selection something that can be implemented without backend authors needing to think about it, so it would be useful if there were support for further partitioning each partition (e.g. by environment field on targets), such that we can dispatch each sub-partition to the appropriate environment.

[danxmoran]

This would also be nice for test - at the moment the core logic asserts that all elements of a batch belong to the same environment, which is the opposite of making it so plugin authors don't have to think about it 😬