Refactor fetchHex out of buildHex.

This will allow to reuse this code to fetch rebar3's dependencies to
ensure it's hermetic build.

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

@@ -1,4 +1,4 @@
-{ stdenv, erlang, rebar3, openssl, libyaml, fetchurl, fetchFromGitHub,
+{ stdenv, erlang, rebar3, openssl, libyaml, fetchHex, fetchFromGitHub,
   rebar3-pc }:
 
 { name, version, sha256
@@ -16,6 +16,12 @@ stdenv.mkDerivation (attrs // {
 
   buildInputs = buildInputs ++ [ erlang rebar3 openssl libyaml ];
 
+  src = fetchHex {
+    pkg = hexPkg;
+    inherit version;
+    inherit sha256;
+  };
+
   postPatch = let
     registrySnapshot = import ./registrySnapshot.nix { inherit fetchFromGitHub; };
   in ''
@@ -35,12 +41,6 @@ stdenv.mkDerivation (attrs // {
     ${postPatch}
   '';
 
-  unpackCmd = ''
-    tar -xf $curSrc contents.tar.gz
-    mkdir contents
-    tar -C contents -xzf contents.tar.gz
-  '';
-
   configurePhase = let
     plugins = pluginDeps ++ (if compilePorts then [rebar3-pc] else []);
     getDeps = drv: [drv] ++ (map getDeps drv.erlangDeps);
@@ -84,11 +84,6 @@ stdenv.mkDerivation (attrs // {
     runHook postInstall
   '';
 
-  src = fetchurl {
-    url = "https://s3.amazonaws.com/s3.hex.pm/tarballs/${hexPkg}-${version}.tar";
-    sha256 = sha256;
-  };
-
   meta = {
     inherit (erlang.meta) platforms;
   } // meta;

pkgs/development/tools/build-managers/rebar3/fetch-hex.nix

@@ -0,0 +1,34 @@
+{ stdenv, fetchurl }:
+
+{ pkg, version, sha256
+, meta ? {}
+}:
+
+with stdenv.lib;
+
+stdenv.mkDerivation ({
+  name = "hex-source-${pkg}-${version}";
+
+  src = fetchurl {
+    url = "https://s3.amazonaws.com/s3.hex.pm/tarballs/${pkg}-${version}.tar";
+    inherit sha256;
+  };
+
+  phases = [ "unpackPhase" "installPhase" ];
+
+  unpackCmd = ''
+    tar -xf $curSrc contents.tar.gz
+    mkdir contents
+    tar -C contents -xzf contents.tar.gz
+  '';
+
+  installPhase = ''
+    runHook preInstall
+    mkdir "$out"
+    cp -Hrt "$out" .
+    success=1
+    runHook postInstall
+  '';
+
+  inherit meta;
+})

pkgs/top-level/all-packages.nix

@@ -5020,6 +5020,7 @@ let
 
   rebar = callPackage ../development/tools/build-managers/rebar { };
   rebar3 = callPackage ../development/tools/build-managers/rebar3 { };
+  fetchHex = callPackage ../development/tools/build-managers/rebar3/fetch-hex.nix { };
 
   erlangPackages = callPackage ../development/erlang-modules { };