chore: export function needed to implement dead / unused code detection functions outside of Lean core by users

[wvhulle]

This PR makes a single function public so i can use it for dead code detection in a Lean linter I am working on. See the Lint rule at https://codeberg.org/wvhulle/heron/src/branch/main/Heron/Check/UnusedImport.lean

[mathlib-lean-pr-testing]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 508a1132422faa6d0a35a135909ddc0526910783 --onto 0a6c31520b2ebaa6e71227454b46ebfb4986ae7b. You can force Mathlib CI using the force-mathlib-ci label. (2026-05-02 16:46:48)
• ❗ Mathlib CI can not be attempted yet, as the nightly-testing-2026-05-02 tag does not exist there yet. We will retry when you push more commits. If you rebase your branch onto nightly-with-mathlib, Mathlib CI should run now. You can force Mathlib CI using the force-mathlib-ci label. (2026-05-02 18:23:45)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 508a1132422faa6d0a35a135909ddc0526910783 --onto 3fc99eef102549c743c6c63547983319bfae6f01. You can force reference manual CI using the force-manual-ci label. (2026-05-02 16:46:49)
• ❗ Reference manual CI can not be attempted yet, as the nightly-testing-2026-05-02 tag does not exist there yet. We will retry when you push more commits. If you rebase your branch onto nightly-with-manual, reference manual CI should run now. You can force reference manual CI using the force-manual-ci label. (2026-05-02 18:23:47)