* Add PURLTypeToOSV with ok flag for unknown types

EcosystemToOSV returns its input unchanged on a miss, so a caller
emitting OSV records can't tell a recognised ecosystem from an unknown
one and ends up writing a value the OSV schema rejects. PURLTypeToOSV
takes the PURL type directly (skip the normalisation step when you
already have a parsed PURL) and returns ok=false on a miss so the
caller can fall back to a GIT range instead.

* Address review: doc comment grammar, name empty-string subtest