Use symlinks for directories entries in cache (#1037)
## Summary
One problem we have in the cache today is that we can't overwrite
entries atomically, because we store unzipped _directories_ in the cache
(which makes installation _much_ faster than storing zipped
directories). So, if you ignore the existing contents of the cache when
writing, you might run into an error, because you might attempt to write
a directory where a directory already exists.
This is especially annoying for cache refresh, because in order to
refresh the cache, we have to purge it (i.e., delete a bunch of stuff),
which is also highly unsafe if Puffin is running across multiple threads
or multiple processes.
The solution I'm proposing here is that whenever we persist a
_directory_ to the cache, we persist it to a special "archive" bucket.
Then, within the other buckets, directory entries are actually symlinks
into that "archive" bucket. With symlinks, we can atomically replace,
which means we can easily overwrite cache entries without having to
delete from the cache.
The main downside is that we'll now accumulate dangling entries in the
"archive" bucket, and so we'll need to implement some form of garbage
collection to ensure that we remove entries with no symlinks. Another
downside is that cache reads and writes will be a bit slower, since we
need to deal with creating and resolving these symlinks.
As an example... after this change, the cache entry for this unzipped
wheel is actually a symlink:
Then, within the archive directory, we actually have two unique entries
(since I intentionally ran the command twice to ensure overwrites were
safe):
