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.

napalm_standard/ratchet.py

@@ -152,6 +152,10 @@ def _lines(source: str) -> int:
 
 def measure(repo: pathlib.Path, package: str) -> dict[str, Any]:
     """Today's excesses, keyed the way the baseline stores them."""
+    if not (repo / package).is_dir():
+        raise FileNotFoundError(
+            f"no package directory {package}/ to measure; name it in [tool.napalm-standard] package"
+        )
     function_lines: dict[str, int] = {}
     function_complexity: dict[str, int] = {}
     python_file_lines: dict[str, int] = {}