This adds changes to the rebar3 expression that patch rebar3 to force it
to be hermetic. Now, by default, rebar3 literally can't download
anything. A 'rebar3-open' expression was added for those folks whe want
the normal rebar3.
This commit adds 187 packages from Hex.pm and documents 100 more that
could not be imported for various reasons. The packages where generated
by hex2nix.
This commit moves all the hex based packages to a single namespace. It
also moves all the packages to a single file. This is in preparation
for the move to a system to generate the hex packages from the hex
package store.
