Combining the concepts of semver and calver. Give semcal a semver and it'll tell you what it's calver would be.
Do an HTTP GET (e.g. in a browser) a URL of the form:
<language>.semcal.org/<package>[==<semver>]
and get back a calver string.
e.g.:
python.semcal.org/requests==2.21.0
returns a calver version string:
2018.12
If you don't provide a ==<semver>, the most recent version will be used.