* Enable storage retention for npm
npm stores packages under the PURL purl builds from the full name, so package rules use the default mapping. Both pkg:npm/@babel/core and pkg:npm/%40babel/core work.
Refs #306
* Tighten npm retention tests and docs
The docs now say npm package rules match the name exactly, so pkg:npm/jsonstream doesn't cover JSONStream. The refetch, package rule and mirror tests were weaker than their names and now check what they claim.
Refs #306
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Co-authored-by: Andrew Nesbitt <andrewnez@gmail.com>
