Replace use_pack, layouts/theme and @theme by new helpers

Packs are now loaded from views, just like upstream, and are
identified by their filenames. The definition of `theme.yml` has
changed as such:
- `pack_directory` is now required
- `pack` is now unused
- `signed_in_preload` has been introduced

config/webpack/shared.js

@@ -1,36 +1,26 @@
 // Note: You must restart bin/webpack-dev-server for changes to take effect
 
-const { resolve } = require('path');
+const { basename, dirname, join, relative, resolve } = require('path');
 
 const CircularDependencyPlugin = require('circular-dependency-plugin');
+const { sync } = require('glob');
 const MiniCssExtractPlugin = require('mini-css-extract-plugin');
+const extname = require('path-complete-extname');
 const webpack = require('webpack');
 const AssetsManifestPlugin = require('webpack-assets-manifest');
 
 const { env, settings, flavours, output } = require('./configuration');
 const rules = require('./rules');
 
-function reducePacks (data, into = {}) {
-  if (!data.pack) return into;
+const extensionGlob = `**/*{${settings.extensions.join(',')}}*`;
 
-  for (const entry in data.pack) {
-    const pack = data.pack[entry];
-    if (!pack) continue;
+function reduceFlavourPacks(data, into = {}) {
+  const packPaths = sync(join(data.pack_directory, extensionGlob));
 
-    let packFiles = [];
-    if (typeof pack === 'string')
-      packFiles = [pack];
-    else if (Array.isArray(pack))
-      packFiles = pack;
-    else
-      packFiles = [pack.filename];
-
-    if (packFiles) {
-      into[`flavours/${data.name}/${entry}`] = packFiles.map(packFile => resolve(data.pack_directory, packFile));
-    }
-  }
-
-  if (!data.name) return into;
+  packPaths.forEach((entry) => {
+    const namespace = relative(join(data.pack_directory), dirname(entry));
+    into[`flavours/${data.name}/${join(namespace, basename(entry, extname(entry)))}`] = resolve(entry);
+  });
 
   for (const skinName in data.skin) {
     const skin = data.skin[skinName];
@@ -43,7 +33,7 @@ function reducePacks (data, into = {}) {
 }
 
 const entries = Object.assign(
-  Object.values(flavours).reduce((map, data) => reducePacks(data, map), {}),
+  Object.values(flavours).reduce((map, data) => reduceFlavourPacks(data, map), {}),
 );