This moves `FileSystem::parent_path` (which takes a `FileSystemPathVc`) to be directly on `FileSystemPathVc`. There's already a `FileSystemPathVc::parent` method which internally calls `FileSystem::parent_path` with self, which seems to be an indirect way to implement this.

With the new method location, we're able to take advantage of `FileSystemPathVc::new_normalized` to skip some unnecessary work. And we also reduce duplication, there were two nearly identical implementations of `parent_path` between `DevServerFileSystem` and `DiskFileSystem`.

… not (#264)

This makes turbopack automatically create `app/layout.js` if
`app/page.js` exists, or `app/layout.tsx` if `app/page.tsx` exists, and
neither layout file already exists.

**Note that I'd prefer this issue to have info severity, but even with a
higher log level of info, the issue is never shown**. Filed #265 to
track this.

Test Plan:
In a new `create-next-app`, opt into appDirectory and create
`app/page.js`. Run turbopack and verify:
* [x] the index page renders correctly
* [x] An issue with Warning level is logged notifying the user an
`app/layout.js` was created
* [x] `app/layout.js` is actually created with basic root layout

In a new `create-next-app`, opt into appDirectory and create
`app/page.tsx`. Run turbopack and verify:
* [x] the index page renders correctly
* [x] An issue with Warning level is logged notifying the user an
`app/layout.tsx` was created
* [x] `app/layout.tsx` is actually created with basic root layout

In a new `create-next-app`, opt into appDirectory and create
`app/page.tsx`. Also create a basic `app/layout.js` (note js layout with
tsx page). Add a custom `<title>` to the layout. Run turbopack and
verify:
* [x] the index page renders correctly
* [x] No issue is logged
* [x] No files are modified
* [x] The custom `<title>` is reflected.

… not (vercel/turborepo#264)

This makes turbopack automatically create `app/layout.js` if
`app/page.js` exists, or `app/layout.tsx` if `app/page.tsx` exists, and
neither layout file already exists.

**Note that I'd prefer this issue to have info severity, but even with a
higher log level of info, the issue is never shown**. Filed vercel/turborepo#265 to
track this.

Test Plan:
In a new `create-next-app`, opt into appDirectory and create
`app/page.js`. Run turbopack and verify:
* [x] the index page renders correctly
* [x] An issue with Warning level is logged notifying the user an
`app/layout.js` was created
* [x] `app/layout.js` is actually created with basic root layout

In a new `create-next-app`, opt into appDirectory and create
`app/page.tsx`. Run turbopack and verify:
* [x] the index page renders correctly
* [x] An issue with Warning level is logged notifying the user an
`app/layout.tsx` was created
* [x] `app/layout.tsx` is actually created with basic root layout

In a new `create-next-app`, opt into appDirectory and create
`app/page.tsx`. Also create a basic `app/layout.js` (note js layout with
tsx page). Add a custom `<title>` to the layout. Run turbopack and
verify:
* [x] the index page renders correctly
* [x] No issue is logged
* [x] No files are modified
* [x] The custom `<title>` is reflected.

… not (vercel/turborepo#264)

This makes turbopack automatically create `app/layout.js` if
`app/page.js` exists, or `app/layout.tsx` if `app/page.tsx` exists, and
neither layout file already exists.

**Note that I'd prefer this issue to have info severity, but even with a
higher log level of info, the issue is never shown**. Filed vercel/turborepo#265 to
track this.

Test Plan:
In a new `create-next-app`, opt into appDirectory and create
`app/page.js`. Run turbopack and verify:
* [x] the index page renders correctly
* [x] An issue with Warning level is logged notifying the user an
`app/layout.js` was created
* [x] `app/layout.js` is actually created with basic root layout

In a new `create-next-app`, opt into appDirectory and create
`app/page.tsx`. Run turbopack and verify:
* [x] the index page renders correctly
* [x] An issue with Warning level is logged notifying the user an
`app/layout.tsx` was created
* [x] `app/layout.tsx` is actually created with basic root layout

In a new `create-next-app`, opt into appDirectory and create
`app/page.tsx`. Also create a basic `app/layout.js` (note js layout with
tsx page). Add a custom `<title>` to the layout. Run turbopack and
verify:
* [x] the index page renders correctly
* [x] No issue is logged
* [x] No files are modified
* [x] The custom `<title>` is reflected.