pathtools: add Prefixes helper function (#2152)

**What type of PR is this?**

> Feature

**What package or component does this PR mostly affect?**

> pathtools

**What does this PR do? Why is it needed?**

In the fix for #2132, I'll need to iterate over the prefixes of a given
slash-separated path. Rather than duplicate the logic from Walk2,
let's extract it into pathtools.

Prefixes returns an iterator over prefixes to avoid needing to allocate
a slice. The code should be much simpler after we adopt Go 1.23 and can
use range-over-iterator loops.

**Which issues(s) does this PR fix?**

Fixes #2132

**Other notes for review**