Instead fetching and resolving src deps (which could depend on pkg deps)
and then pkg deps this patch combines the two into a single set of
iterations by level. The only difference between src and pkg deps in this
new install_deps is how their deps list is found -- from the config or
lock file for src deps and from the neighbors of the vertex for pkg.

- 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