Related NEXT-472
When you have `app/page.js` and `pages/page.js`, the `pagePath` are the
same, which is `/page`. This will result require cache conflicts. When
you visit the `app/` page first, then the `pages/` page, the 2nd request
will still get the app dir page module, which result in server error.
Solution: use different cache key of pagePathCache for pages/ and app/
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## Bug
- [x] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Errors have a helpful link attached, see
[`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)
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