extensionUtils: Create and allow access to a new "extension" object

The "extension" object is what I previously called the "helper" object.
It contains the extension importer object as well as the metadata object.
Things that were previously added on to the metadata (state, path, dir, etc.)
are now part of this new "extension" object.

With the new importer changes brought on by the extension prefs tool,
extensions are left without a way to import submodules at the global scope,
which would make them rely on techniques like:

  var MySubModule;

  function init(meta) {
      MySubModule = meta.importer.mySubModule;
  }

That is, there's now a lot more meaningless boilerplate that nobody wants
to write and nobody wants to reivew.

Let's solve this with a few clever hacks.
Allow extensions to get their current extension object with:

  let extension = imports.misc.extensionUtils.getCurrentExtension();

As such, extensions can now get their own extension object before the
'init' method is called, so they can import submodules or do other things
at the module scope:

  const MySubModule = extension.imports.mySubModule;
  const dataPath = GLib.build_filenamev([extension.path, 'awesome-data.json']);

https://bugzilla.gnome.org/show_bug.cgi?id=668429

js/misc/extensionUtils.js

@@ -17,6 +17,38 @@ const ExtensionType = {
 // GFile for user extensions
 var userExtensionsDir = null;
 
+// Maps uuid -> metadata object
+const extensions = {};
+
+function getCurrentExtension() {
+    let stack = (new Error()).stack;
+
+    // Assuming we're importing this directly from an extension (and we shouldn't
+    // ever not be), its UUID should be directly in the path here.
+    let extensionStackLine = stack.split('\n')[1];
+    if (!extensionStackLine)
+        throw new Error('Could not find current extension');
+
+    // The stack line is like:
+    //   init([object Object])@/home/user/data/gnome-shell/extensions/u@u.id/prefs.js:8
+    //
+    // In the case that we're importing from
+    // module scope, the first field is blank:
+    //   @/home/user/data/gnome-shell/extensions/u@u.id/prefs.js:8
+    let match = new RegExp('@(.+):\\d+').exec(extensionStackLine);
+    if (!match)
+        throw new Error('Could not find current extension');
+
+    let path = match[1];
+    let uuid = GLib.path_get_basename(GLib.path_get_dirname(path));
+
+    let extension = extensions[uuid];
+    if (extension === undefined)
+        throw new Error('Could not find current extension');
+
+    return extension;
+}
+
 /**
  * versionCheck:
  * @required: an array of versions we're compatible with
@@ -47,17 +79,17 @@ function versionCheck(required, current) {
     return false;
 }
 
-function isOutOfDate(meta) {
-    if (!versionCheck(meta['shell-version'], Config.PACKAGE_VERSION))
+function isOutOfDate(extension) {
+    if (!versionCheck(extension.metadata['shell-version'], Config.PACKAGE_VERSION))
         return true;
 
-    if (meta['js-version'] && !versionCheck(meta['js-version'], Config.GJS_VERSION))
+    if (extension.metadata['js-version'] && !versionCheck(extension.metadata['js-version'], Config.GJS_VERSION))
         return true;
 
     return false;
 }
 
-function loadMetadata(uuid, dir, type) {
+function createExtensionObject(uuid, dir, type) {
     let info;
 
     let metadataFile = dir.get_child('metadata.json');
@@ -95,21 +127,27 @@ function loadMetadata(uuid, dir, type) {
         throw new Error('uuid "' + meta.uuid + '" from metadata.json does not match directory name "' + uuid + '"');
     }
 
-    meta.type = type;
-    meta.dir = dir;
-    meta.path = dir.get_path();
-    meta.error = '';
-    meta.hasPrefs = dir.get_child('prefs.js').query_exists(null);
+    let extension = {};
 
-    return meta;
+    extension.metadata = meta;
+    extension.uuid = meta.uuid;
+    extension.type = type;
+    extension.dir = dir;
+    extension.path = dir.get_path();
+    extension.error = '';
+    extension.hasPrefs = dir.get_child('prefs.js').query_exists(null);
+
+    extensions[uuid] = extension;
+
+    return extension;
 }
 
-var _meta = null;
+var _extension = null;
 
-function installImporter(meta) {
-    _meta = meta;
-    ShellJS.add_extension_importer('imports.misc.extensionUtils._meta', 'importer', meta.path);
-    _meta = null;
+function installImporter(extension) {
+    _extension = extension;
+    ShellJS.add_extension_importer('imports.misc.extensionUtils._extension', 'imports', extension.path);
+    _extension = null;
 }
 
 function init() {