rez package cache stalls on osx

I was having problems getting the package cache to work under osx.
I've set my cache folder to /Libraries/Cache/rez/package_cache.
When resolving a rez env, the rez-pkg-cache shows the cache as `stalled`.
On inspecting the cache folder, it looks like the copy succeeded, but the final removal of the .copying file has failed.
I've had some issues with shutil on osx, so thought I'd replace it with popen and normal shell `cp` command.
Having done this, the cache is being made correctly without stalling.

Start by reproducing the stalled rez-pkg-cache state on OSX with the cache configured at /Libraries/Cache/rez/package_cache, then inspect the cache operation that leaves the .copying file behind. Done means the package copy completes, the temporary marker is removed, and the cache is no longer reported as stalled.

Written by the indexing model from the issue text.