fix: only pass walked dir content to single callback (#2134)

https://github.com/bazel-contrib/bazel-gazelle/commit/1925a30534a0ab9adaa7afd772d335e3720b1abe
(https://github.com/bazel-contrib/bazel-gazelle/pull/2109) broke
`generation_mode`, basically ignoring the mode and invoking on
directories that are `containedByParent`

**What type of PR is this?**

Bug fix

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

all

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

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

Fixes #

**Other notes for review**