Add a Pug plugin to allow multiple basedirs.

This will make it possible to cleanly separate Bedrock's templates from
the user project templates.

This is cherry-picked from
usebedrock/bedrock#404.

core/templates/multi-basedirs.js

@@ -0,0 +1,53 @@
+/*
+ * This is a Pug plugin to allow multiple basedirs. This is used to have Pug
+ * templates in both Bedrock core and in a project that uses Bedrock.
+ *
+ * From https://github.com/pugjs/pug/issues/2499 and
+ * https://github.com/gryphonmyers/pug-multiple-basedirs-plugin
+ */
+
+const { accessSync, constants: {R_OK} } = require('fs');
+const path = require('path');
+
+const config = require('../discovery/config');
+
+// The multiple basedirs are the user-defined basedir plus Bedrock's. Adding
+// bedrockDir allows to refer to Bedrock's core templates with the /core
+// prefix. Adding it at the end allows users to override core templates.
+const bedrockDir = path.join(__dirname, '../../');
+const basedirs = [config.pug.basedir, bedrockDir];
+
+const exists = filename => {
+  try {
+    accessSync(filename, R_OK);
+    return true;
+  } catch (err) {
+    return false;
+  }
+};
+
+function MultipleBaseDirs() {
+  return {
+    name: 'multipleBasedirs',
+    resolve(filename, source) {
+      let out;
+
+      if (filename[0] === '/') {
+        filename = filename.substr(1);
+        if (!basedirs.some(basedir => exists(out = path.resolve(basedir, filename)))) {
+          throw new Error(`${filename} cannot be found in any basedir`);
+        }
+      } else {
+        if (!source) {
+          throw new Error('the "filename" option is required to use includes and extends with "relative" paths');
+        }
+
+        out = path.resolve(path.dirname(source), filename);
+      }
+
+      return out;
+    }
+  };
+}
+
+module.exports = MultipleBaseDirs;