fix($core): normalize theme path with Layout.vue correctly (close vuejs#2178)

Author: meteorlxy

Diff for: packages/@vuepress/core/lib/node/loadTheme.js

@@ -60,8 +60,15 @@ module.exports = function loadTheme (ctx) {
 }
 
 function normalizeThemePath (resolved) {
-  const { entry, fromDep } = resolved
+  const { entry, name, fromDep } = resolved
   if (fromDep) {
+    const packageRoot = require.resolve(`${name}/package.json`)
+    const { main } = require(packageRoot)
+    if (main.endsWith('.vue')) {
+      // For those cases that "main" field is set to .vue file
+      // e.g. `layouts/Layout.vue`
+      return parse(packageRoot).dir
+    }
     return parse(require.resolve(entry)).dir
   } else if (entry.endsWith('.js') || entry.endsWith('.vue')) {
     return parse(entry).dir