Consistent recursive cp, get and put into non-existent target directory

[ianthomas23]

This is a WIP for #1062. Running the test script from that issue gives the following output:

==> cp
  loop 0: ['/tmp/target/a']
  loop 1: ['/tmp/target/a', '/tmp/target/source', '/tmp/target/source/a']
==> cp, slash
  loop 0: ['/tmp/target/a']
  loop 1: ['/tmp/target/a']
==> rsync
  loop 0: ['/tmp/target/source', '/tmp/target/source/a']
  loop 1: ['/tmp/target/source', '/tmp/target/source/a']
==> rsync, slash
  loop 0: ['/tmp/target/a']
  loop 1: ['/tmp/target/a']
==> scp
  loop 0: ['/tmp/target/a']
  loop 1: ['/tmp/target/a', '/tmp/target/source', '/tmp/target/source/a']
==> scp, slash
  loop 0: ['/tmp/target/a']
  loop 1: ['/tmp/target/a']
==> fs.cp
  loop 0: ['/tmp/target/a']
  loop 1: ['/tmp/target/a', '/tmp/target/source', '/tmp/target/source/a']
==> fs.cp, slash
  loop 0: ['/tmp/target/a']
  loop 1: ['/tmp/target/a']
==> fs.get
  loop 0: ['/tmp/target/a']
  loop 1: ['/tmp/target/a', '/tmp/target/source', '/tmp/target/source/a']
==> fs.get, slash
  loop 0: ['/tmp/target/a']
  loop 1: ['/tmp/target/a']
==> fs.put
  loop 0: ['/tmp/target/a']
  loop 1: ['/tmp/target/a', '/tmp/target/source', '/tmp/target/source/a']
==> fs.put, slash
  loop 0: ['/tmp/target/a']
  loop 1: ['/tmp/target/a']
==> mem.cp
  loop 0: ['/remote_target/a']
  loop 1: ['/remote_target/a', '/remote_target/remote_source', '/remote_target/remote_source/a']
==> mem.cp, slash
  loop 0: ['/remote_target/a']
  loop 1: ['/remote_target/a']
==> mem.get
  loop 0: ['/tmp/target/a']
  loop 1: ['/tmp/target/a', '/tmp/target/remote_source', '/tmp/target/remote_source/a']
==> mem.get, slash
  loop 0: ['/tmp/target/a']
  loop 1: ['/tmp/target/a']
==> mem.put
  loop 0: ['/remote_target/a']
  loop 1: ['/remote_target/a', '/remote_target/source', '/remote_target/source/a']
==> mem.put, slash
  loop 0: ['/remote_target/a']
  loop 1: ['/remote_target/a']

showing that the fsspec functions cp, get and put are consistent with command-line cp and scp.

The solution is mostly about passing the correct arguments to utils.other_paths() to obtain the required behaviour.

It is only a WIP for two reasons:

1. I have broken the use case of fs.copy(['/tmp/source/a', '/tmp/source/b'], '/tmp/target') and don't yet have a fix for it.
2. I have only fixed other_paths() calls in spec.py not in asyn.py, which presumably needs similar logic.

[ianthomas23]

This now works for the synchronous case, including for copying 2+ named files. I still need to update and test the async side of things.

[martindurant]

All looks good!
One question.