Keep the identity of listed keys through traversal, fan-outs, and invalidation #1381

Part of #1341 (decisions 7, 8 and the key="" part of 10 of the audit). Defect 23 of #1327 and the items deferred from #1370.

Start after #1344 merges, since both rewrite the pairing and the fan-outs of rm, chmod and copy/mv.

Current state (29cc1d3)

1. Listed names are re-parsed as paths. With the literal keys a/x and a/x?versionId=null in an unversioned bucket, find("bucket/a") names the second bucket/a/x?versionId=null, and:
   • chmod(recursive=True) updates a/x twice (once with VersionId=null) and never the literal key;
   • rm(recursive=True) deletes a/x and a/x version null, and leaves the literal key;
   • copy(recursive=True) raises ValueError: Cannot copy to a versioned file. after copying a/x;
   • mv(recursive=True) raises ValueError: Cannot move several paths to the same destination. before copying;
   • fsspec's du counts a/x once.
2. Traversal and invalidation. With the key a/dir?versionId=v1/z, find("bucket/a") lists it but find("bucket/a", maxdepth=2) does not: _find_levels (s3.py:930) re-parses the directory name. _invalidate_path (s3.py:2594) can take part of such a key for a version and skip the parent's listing.
3. bucket//. S3Path.parse("b//") is the key / with is_bucket True. find and recursive chmod list the bucket, and the recursive chmod sends no PutBucketAcl; rm_file("bucket//") deletes the key /.
4. key="". S3Path("b", "") prints b and re-parses with key None.

Decision

1. Filesystem operations carry the bucket, key and version ID of the listed entries through traversal, pairing, the fan-outs, lookups and invalidation, as a private sequence of typed paths; they never re-parse a listed name. Public find() and expand_path() still return strings.
2. A key that looks like a version path cannot be addressed by a string path. This stays a documented limit, as in s3fs; S3Core with an explicit S3Path reaches it. No escape syntax.
3. bucket// keeps its current meaning (the key / for single-object operations, the bucket for listings), documented, including that a recursive chmod of it does not change the bucket ACL.
4. S3Path(bucket, "") normalizes the key to None.