refactor: Reshuffle file structure for better code layout

This gets rid of the package called "server" and instead moves
everything into the project root, such that Go actually builds us a
binary called `nixery`.

This is the first step towards factoring out CLI-based functionality
for Nixery.

tools/nixery/prepare-image/load-pkgs.nix

@@ -0,0 +1,45 @@
+# Copyright 2019 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# Load a Nix package set from one of the supported source types
+# (nixpkgs, git, path).
+{ srcType, srcArgs, importArgs ? { } }:
+
+with builtins;
+let
+  # If a nixpkgs channel is requested, it is retrieved from Github (as
+  # a tarball) and imported.
+  fetchImportChannel = channel:
+    let
+      url =
+        "https://github.com/NixOS/nixpkgs-channels/archive/${channel}.tar.gz";
+    in import (fetchTarball url) importArgs;
+
+  # If a git repository is requested, it is retrieved via
+  # builtins.fetchGit which defaults to the git configuration of the
+  # outside environment. This means that user-configured SSH
+  # credentials etc. are going to work as expected.
+  fetchImportGit = spec: import (fetchGit spec) importArgs;
+
+  # No special handling is used for paths, so users are expected to pass one
+  # that will work natively with Nix.
+  importPath = path: import (toPath path) importArgs;
+in if srcType == "nixpkgs" then
+  fetchImportChannel srcArgs
+else if srcType == "git" then
+  fetchImportGit (fromJSON srcArgs)
+else if srcType == "path" then
+  importPath srcArgs
+else
+  throw ("Invalid package set source specification: ${srcType} (${srcArgs})")