refactor: simplify walkDir signature (#2043)

There's 2 commits here both simplifying it in different ways.

**What type of PR is this?**

Other

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

all

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

Cleanup regarding
https://github.com/bazel-contrib/bazel-gazelle/pull/2039#discussion_r1964845284