* use hex_core tarball unpacking support in pkg resource
* ignore etag if package doesn't exist and delete if checksum fails
* add back tests for bad package checksums
* improve bad registry checksum error message
- The rebar package index files have been moved off the default path and
will require a new `rebar3 update`
- Caching of downloaded packages automatically takes place in a path
relative to the CDN used
- The cache path is not shared with hex as we now write and modify data
in there arbitrarily
- Basic tests plus the working set for more of them is included
Tristan Sloughter
Fred Hebert