fix($core): failed to load theme when using "layouts/Layout.vue" as entry file...

fix($core): failed to load theme when using "layouts/Layout.vue" as entry file (close: #1563) (#1564)

__mocks__/vuepress-theme-without-index/index.js

+module.exports = {}
+// This file is just for jest.

__mocks__/vuepress-theme-without-index/package.json

+{
+    "name": "vuepress-theme-without-index",
+    "main": "Layout.vue"
+}

packages/@vuepress/core/lib/node/__tests__/prepare/App.spec.js

@@ -18,6 +18,7 @@ describe('App', () => {
         theme: '@vuepress/default',
         emp: docsTempPath
       })
+      await app.process()
       expect(app.sourceDir).toBe(docsPath)
     }))
   })

packages/@vuepress/core/lib/node/__tests__/prepare/fixtures/docs-dep-theme-without-index/.vuepress/config.js

+module.exports = {
+    title: 'Hello VuePress',
+    description: '# Hello, VuePress!',
+    theme: 'vuepress-theme-without-index'
+  }

packages/@vuepress/core/lib/node/__tests__/prepare/fixtures/docs-dep-theme-without-index/README.md

+# Hello, VuePress!

packages/@vuepress/core/lib/node/loadTheme.js

@@ -56,17 +56,7 @@ module.exports = function loadTheme (ctx) {
 function normalizeThemePath (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 an non-index file
-      // e.g. `layouts/Layout.vue`
-      return packageRoot
-    } else {
-      // For those cases that "index.js" is not at package root
-      // e.g. `lib/index.js` (#1362)
-      return parse(require.resolve(name)).dir
-    }
+    return parse(require.resolve(name)).dir
   } else if (entry.endsWith('.js') || entry.endsWith('.vue')) {
     return parse(entry).dir
   } else {