erlang: refactor: build packages per Erlang/OTP.

This change introduces a fixpoint, which allows to do deep override when
building packages defined in pkgs/development/beam-modules/default.hex.
This allows to provide beam.packages.erlang{,R16,R17,R18,R19} which
contains the same packages built with different Erlang/OTP versions.

Top-level attribute beamPackages points at beam.packages.erlangR18, the
same applies to other top-level Erlang packages.

TODO:
- beam.packages.erlang{R16,R17} is almost useless, since rebar/rebar3
  does not build using these versions;
- all packages in beam.packages which use buildMix are actually built
  with erlangR18;
- update documentation.

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

@@ -1,13 +1,14 @@
 { stdenv, buildRebar3, fetchHex }:
 
 { name, version, sha256
+, builder ? buildRebar3
 , hexPkg ? name
 , ... }@attrs:
 
 with stdenv.lib;
 
 let
-  pkg = self: buildRebar3 (attrs // {
+  pkg = self: builder (attrs // {
 
     src = fetchHex {
       pkg = hexPkg;

pkgs/development/beam-modules/default.nix

@@ -1,16 +1,45 @@
-{ stdenv, pkgs }:
+{ stdenv, pkgs, erlang, overrides ? (self: super: {}) }:
 
 let
-  self = rec {
-    hexPackages = import ./hex-packages.nix { stdenv = stdenv; callPackage = self.callPackage; pkgs = pkgs; };
-    callPackage = pkgs.lib.callPackageWith (pkgs // self // hexPackages);
-    buildRebar3 = callPackage ./build-rebar3.nix {};
-    buildHex = callPackage ./build-hex.nix {};
-    buildErlangMk = callPackage ./build-erlang-mk.nix {};
-    buildMix = callPackage ./build-mix.nix {};
+  inherit (stdenv.lib) fix' extends;
 
-    ## Non hex packages
-    hex = callPackage ./hex {};
-    webdriver = callPackage ./webdriver {};
-  };
-in self // self.hexPackages
+  # FIXME: add support for overrideScope
+  callPackageWithScope = scope: drv: args: stdenv.lib.callPackageWith scope drv args;
+  mkScope = scope: pkgs // scope;
+
+  packages = self:
+    let
+      defaultScope = mkScope self;
+      callPackage = drv: args: callPackageWithScope defaultScope drv args;
+    in
+      import ./hex-packages.nix {
+        inherit pkgs stdenv callPackage;
+      } // {
+        inherit callPackage erlang;
+        beamPackages = self;
+
+        rebar = callPackage ../tools/build-managers/rebar { };
+        rebar3-open = callPackage ../tools/build-managers/rebar3 {
+          hermeticRebar3 = false;
+        };
+        rebar3 = callPackage ../tools/build-managers/rebar3 {
+          hermeticRebar3 = true;
+        };
+
+        hexRegistrySnapshot = callPackage ./hex-registry-snapshot.nix { };
+        fetchHex = callPackage ./fetch-hex.nix { };
+
+        buildRebar3 = callPackage ./build-rebar3.nix {};
+        buildHex = callPackage ./build-hex.nix {};
+        buildErlangMk = callPackage ./build-erlang-mk.nix {};
+        buildMix = callPackage ./build-mix.nix {};
+
+        ## Non hex packages
+        hex = callPackage ./hex {};
+        webdriver = callPackage ./webdriver {};
+
+        hex2nix = callPackage ../tools/erlang/hex2nix { };
+        cuter = callPackage ../tools/erlang/cuter { };
+        relxExe = callPackage ../tools/erlang/relx-exe {};
+      };
+in fix' (extends overrides packages)