fix(ratchet): refuse a package directory that does not exist

napalm-hpe-aruba-procurve ships napalm_procurve; guessed from its name, the
ratchet measured nothing and wrote an empty baseline without a word.

tests/test_ratchet.py

@@ -308,3 +308,15 @@ def test_check_fails_on_a_problem_and_on_a_stale_entry(tmp_path: pathlib.Path) -
     assert code == 1
     assert "pkg/mod.py::long = 62 (new)" in text
     assert "stale: function_lines: pkg/mod.py::old" in text
+
+
+def test_a_package_that_does_not_exist_is_an_error_not_an_empty_baseline(
+    tmp_path: pathlib.Path,
+) -> None:
+    """procurve's import package is napalm_procurve: guessed from the name, the
+    ratchet measured nothing and wrote an empty baseline without a word."""
+    repo = _repo(tmp_path, LONG)
+
+    with pytest.raises(FileNotFoundError, match="napalm_missing"):
+        qr.run(repo, "napalm_missing", write=True)
+    assert not (repo / qr.BASELINE).exists()