Move registry to rebar3, since it needs it itself to be bootstrapped.

This makes rebar3 build a bit more hermetic. Next stage is to pull in
rebar3 plugins dependencies for bootstrap to pick up.

pkgs/development/erlang-modules/build-hex.nix

@@ -22,21 +22,17 @@ stdenv.mkDerivation (attrs // {
     inherit sha256;
   };
 
-  postPatch = let
-    registrySnapshot = import ./registrySnapshot.nix { inherit fetchFromGitHub; };
-  in ''
+  postPatch = ''
     rm -f rebar rebar3
     if [ -e "src/${name}.app.src" ]; then
       sed -i -e 's/{ *vsn *,[^}]*}/{vsn, "${version}"}/' "src/${name}.app.src"
     fi
 
-    # TODO: figure out how to provide 'pc' plugin hermetically
     ${if compilePorts then ''
     echo "{plugins, [pc]}." >> rebar.config
     '' else ''''}
 
-    mkdir -p _build/default/{lib,plugins}/ ./.cache/rebar3/hex/default/
-    zcat ${registrySnapshot}/registry.ets.gz > .cache/rebar3/hex/default/registry
+    ${rebar3.setupRegistry}
 
     ${postPatch}
   '';
@@ -61,8 +57,6 @@ stdenv.mkDerivation (attrs // {
     runHook postConfigure
   '';
 
-  # TODO: figure out how to provide rebar3 a static registry snapshot to make
-  # this hermetic
   buildPhase = ''
     runHook preBuild
     HOME=. rebar3 compile

pkgs/development/tools/build-managers/rebar3/default.nix

@@ -1,8 +1,13 @@
-{ stdenv, fetchurl, erlang }:
+{ stdenv, fetchurl, erlang, tree, fetchFromGitHub }:
 
 
 let
   version = "3.0.0-beta.4";
+  registrySnapshot = import ./registrySnapshot.nix { inherit fetchFromGitHub; };
+  setupRegistry = ''
+    mkdir -p _build/default/{lib,plugins,packages}/ ./.cache/rebar3/hex/default/
+    zcat ${registrySnapshot}/registry.ets.gz > .cache/rebar3/hex/default/registry
+  '';
 in
 stdenv.mkDerivation {
   name = "rebar3-${version}";
@@ -13,8 +18,11 @@ stdenv.mkDerivation {
   };
 
   buildInputs = [ erlang ];
+  inherit setupRegistry;
+
 
   buildPhase = ''
+    ${setupRegistry}
     HOME=. escript bootstrap
   '';
   installPhase = ''