ETags should be stored in rebar_pkg_resource

[tothlac]

Currently ETags are calculated from the pkg file stored locally, which is wrong.

As mentioned by @ericmj in previous issue: #1741, the ETags
sent back from the hex server should be stored together with the downloaded pkg files.

A possible solution is to store etags in a .eterm file in ~/.cache/rebar3/hex/default/packages/pkg_etags.eterm in proplist format.

What do you think about this solution?

[ferd]

It should be cheaper to run than the current thing, even though the bottleneck will remain the hash check. I don't have much of a problem with caching the etag, though I would probably store them as file pairs rather than one big etag file + 50 packages. I.e. package-vsn.tgz + package-vsn.etag ?

[tothlac]

I have created this PR:
#1745

[tothlac]

The return values in rebar_pkg_resource were a bit confusing for me, so I have created an other pull request containing some type specs for the module: https://github.com/erlang/rebar3/pull/1746/commits

[ferd]

All merged.