HTTP cache: RepositoryResources resource is not cached in a versioned way

[dtivel]

Although RepositorySignatures resources are versioned, the cached content is not versioned in NuGet's HTTP cache.

This means if a 5.0 client populates the cache with a RepositorySignatures/5.0.0 resource where allRepositorySigned is true and then a 4.7-4.9.3 client uses the cached resource, restore will fail like this.

1. Clear all caches. (nuget locals -clear all)
2. Restore a package from https://nuget.org using a 5.0 client.
3. Within 30 minutes, which is how long the HTTP cache is valid, use a 4.9.3 client to restore a different package.

Result: The 4.9.3 client restore will fail like this.