Fix `external` directive to follow symlinks.

[felberj]

This also allows following links pointing to outside of the repository.

What type of PR is this?

Bug fix Feature

What package or component does this PR mostly affect?

walk

What does this PR do? Why is it needed?

This fixes the follow directive by implementing following symlinks.

Which issues(s) does this PR fix?

Fixes #1574

Other notes for review

This also allows following symlinks outside of the repo (this is actually what I want)