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.

CHANGELOG.md

@@ -15,6 +15,9 @@ The format follows [Keep a Changelog](https://keepachangelog.com/), versions fol
 ### Fixed
 - The shared workflow waits 60 s per download and retries ten times, so a slow PyPI
   response no longer fails a job.
+- `ratchet` refuses a package directory that does not exist. Guessed from the name
+  (napalm-hpe-aruba-procurve ships `napalm_procurve`), it measured nothing and wrote an
+  empty baseline without a word.
 - `security` audited the runner's own pip instead of the package: pip-audit now
   checks a venv holding only the package and its dependencies.