install, comm: remove file_diff in favor of uucore::fs::are_files_identical

[wtcpython]

Remove unmaintained file_diff dependency.

Moves are_files_identical from uu_comm to uucore::fs, and uses it in uu_install's need_copy.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/tail/tail-n0f is now being skipped but was previously passing.

[codspeed-hq]

Merging this PR will not alter performance

✅ 357 untouched benchmarks
⏩ 50 skipped benchmarks¹

---

_{Comparing wtcpython:remove-file-diff (9364607) with main (8b8cbfd)}

Footnotes

1. 50 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[sylvestre]

LGTM, thanks