The mirror passed ids through as written, so pkg:nuget/Newtonsoft.Json@13.0.3 was stored under a copy no client is served while the handler kept serving its lowercased one. GetOrFetchArtifact now lowercases nuget names, and the docs no longer ask for lowercase ids in mirror lists.
Refs #306
The handler stores NuGet ids lowercased, but purl keeps nuget names as written, so package rules get lowercased before they're matched. Case in a rule doesn't matter.
The mirror stores ids as given, so a mixed-case PURL ends up as a copy clients never get. Until that's fixed, the docs ask for lowercase ids in mirror lists.
Refs #306
