nix: Add methods to support develop installations from config.

shell.nix

@@ -33,6 +33,39 b' let'
 
   ce-pythonPackages = enterprise-ce.pythonPackages;
 
+  # This method looks up a path from `pkgs.config.rc.sources` and returns a
+  # shell script which does a `python setup.py develop` installation of it. If
+  # no path is found it will return an empty string.
+  optionalDevelopInstall = attributeName:
+    let
+      path = pkgs.lib.attrByPath [attributeName] null sources;
+      doIt = doDevelopInstall && path != null;
+    in
+      pkgs.lib.optionalString doIt (
+      builtins.trace "Develop install of ${attributeName} from ${path}" ''
+        echo "Develop install of '${attributeName}' from '${path}' [BEGIN]"
+        pushd ${path}
+        python setup.py develop --prefix $tmp_path --allow-hosts ""
+        popd
+        echo "Develop install of '${attributeName}' from '${path}' [DONE]"
+      '');
+
+  # This method looks up a path from `pkgs.config.rc.sources` and imports the
+  # default.nix file if it exists. It returns the list of build inputs. If no
+  # path is found it will return an empty list.
+  optionalDevelopInstallBuildInputs = attributeName:
+    let
+      path = pkgs.lib.attrByPath [attributeName] null sources;
+      nixFile = "${path}/default.nix";
+      doIt = doDevelopInstall && path != null && pkgs.lib.pathExists "${nixFile}";
+      derivate = import "${nixFile}" {
+        inherit doCheck pkgs pythonPackages;
+      };
+    in
+      pkgs.lib.lists.optionals doIt derivate.propagatedNativeBuildInputs;
+
+  developInstalls = [ "rhodecode-vcsserver" ];
+
 in enterprise-ce.override (attrs: {
   # Avoid that we dump any sources into the store when entering the shell and
   # make development a little bit more convenient.