Add shell env to buildHex packages.

This adds erlangPackages.${name}.env, which is usable to launch a shell
with the package and it's dependencies like this:

    nix-shell -A erlangPackages.lager.env

This required:

1) refactoring buildHex to become a fixed-point function,
2) moves output of a package into $out/${name},
3) introduces erlEnv (buildEnv, which links in package and it's deps
   into one directory - a super-simple plagiarization of
   haskellPackages.compiler.ghcWithPackages) and
4) shell function producing a un-buildable derivation to be used by
   nix-shell

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

@@ -1,3 +1,6 @@
+# This file is not used not tested at this time, build-hex.nix is the currently
+# main vehicle of bringing Erlang packages in.
+
 { stdenv, erlang, rebar, openssl, libyaml }:
 
 { name, version

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

@@ -1,5 +1,5 @@
 { stdenv, erlang, rebar3, openssl, libyaml, fetchHex, fetchFromGitHub,
-  rebar3-pc }:
+  rebar3-pc, buildEnv }:
 
 { name, version, sha256
 , hexPkg ? name
@@ -11,7 +11,29 @@
 
 with stdenv.lib;
 
-stdenv.mkDerivation (attrs // {
+let
+  plugins = pluginDeps ++ (if compilePorts then [rebar3-pc] else []);
+  getDeps = drv: [drv] ++ (map getDeps drv.erlangDeps);
+  recursiveDeps = unique (flatten (map getDeps erlangDeps));
+  recursivePluginsDeps = unique (flatten (map getDeps plugins));
+
+  erlEnv = drv: buildEnv {
+    name = "erlang-env-${drv.name}";
+    paths = [ drv ] ++ recursiveDeps;
+    ignoreCollisions = false;
+    meta = drv.meta;
+  };
+
+  shell = drv: let
+    drvEnv = erlEnv drv;
+  in stdenv.mkDerivation {
+    name = "interactive-shell-${drv.name}";
+    nativeBuildInputs = [ erlang drvEnv ];
+    shellHook = ''
+      export ERL_LIBS="${drvEnv}";
+    '';
+  };
+  pkg = self: stdenv.mkDerivation (attrs // {
     name = "${name}-${version}";
 
     buildInputs = buildInputs ++ [ erlang rebar3 openssl libyaml ];
@@ -37,21 +59,16 @@ stdenv.mkDerivation (attrs // {
       ${postPatch}
     '';
 
-  configurePhase = let
-    plugins = pluginDeps ++ (if compilePorts then [rebar3-pc] else []);
-    getDeps = drv: [drv] ++ (map getDeps drv.erlangDeps);
-    recursiveDeps = unique (flatten (map getDeps erlangDeps));
-    recursivePluginsDeps = unique (flatten (map getDeps plugins));
-  in ''
+    configurePhase = ''
       runHook preConfigure
       ${concatMapStrings (dep: ''
         header "linking erlang dependency ${dep}"
-      ln -s "${dep}" "_build/default/lib/${dep.packageName}"
+        ln -s "${dep}/${dep.name}" "_build/default/lib/${dep.name}"
         stopNest
       '') recursiveDeps}
       ${concatMapStrings (dep: ''
         header "linking rebar3 plugins ${dep}"
-      ln -s "${dep}" "_build/default/plugins/${dep.packageName}"
+        ln -s "${dep}/${dep.name}" "_build/default/plugins/${dep.name}"
         stopNest
       '') recursivePluginsDeps}
       runHook postConfigure
@@ -68,11 +85,11 @@ stdenv.mkDerivation (attrs // {
 
     installPhase = ''
       runHook preInstall
-    mkdir "$out"
+      mkdir -p "$out/${name}"
       for reldir in src ebin priv include; do
         fd="_build/default/lib/${name}/$reldir"
         [ -d "$fd" ] || continue
-      cp -Hrt "$out" "$fd"
+        cp -Hrt "$out/${name}" "$fd"
         success=1
       done
       runHook postInstall
@@ -84,6 +101,9 @@ stdenv.mkDerivation (attrs // {
 
     passthru = {
       packageName = name;
+      env = shell self;
       inherit erlangDeps;
     };
-})
+  });
+in
+  fix pkg