To successfully build rebar packages, it needs to be provided with
rebar3 plugins used to build it. This change passes them to env
variable. From there rebar3-nix-bootstrap takes them and symlinks into
_build/default/plugins.
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.
Building Hex packages is a superset of building with rebar3. There is no
need to force folks that use rebar3 but not hex to build with hex. This
commit seperates the rebar3 specific bits and the hex specific bits into
seperate functions that can be used independently.
