Pages outside of 'path' are also listed in collections

**Describe the bug**

A folder collection with `path` specified as `{{slug}}/index` also lists pages in `folder` that are not in a subdirectory.

**To Reproduce**
1. Clone repository: https://github.com/lbischof/issue-4884
2. Start a webserver and local proxy server (see README.md)
3. Visit localhost:8080/admin/
4. See that "This should not appear" is also listed.

**Expected behavior**
I expect that if `path` is specified with a `/`, then all posts/pages that do not fulfill this requirement should be ignored and not listed.

**Applicable Versions:**

- Netlify CMS version: 2.10.84

**CMS configuration**
https://github.com/lbischof/issue-4884