mv() rejects a prefix-only source that it would skip #1397

Problem

mv() rejects a move with a prefix-only source that it would skip. With the keys dir/a and x in bucket (no object at dir), at d03e034:

fs.mv(["s3://bucket/dir", "s3://bucket/x"], ["s3://bucket/x", "s3://bucket/y"])
# ValueError: Cannot move a path onto another path that is moved.

dir has no object at its key, so the move would skip it and move x to y. copy() with the same pairs does that since #1344.

Cause

S3PathPairing.conflict_candidates() (s3_path_pairing.py:143) returns, for a move, only the sources that have another source below them in the pairs. Only those are looked up, so a prefix given in a list without its keys is treated as an object, and its destination x conflicts with the source x. For a copy, #1344 made every conflicting source without a version a candidate (copy=True), so the lookup finds that dir is not an object and the pair is skipped.

AioS3FileSystem._mv() pairs its paths with S3FileSystem._move_pairs() and has the same behavior.

Open questions

• Whether mv() uses the candidates of a copy. During Run the batch operations of S3FileSystem concurrently (GH-1322) #1344, doing so made 11 existing tests of mv() fail, so it was left out there; each needs a decision on whether it pins intended behavior.
• The cost: one more lookup for each conflicting source without a version.

Context

Found in #1344 (step 4a of #1332); predates it.