NuGet group repository only shows content already cached in proxies

Might need discussion as to how we want this to behave, but presently queries to a NuGet group do not forward to proxies but only consult already cached data for proxies. This is definitely not the behaviour a client would expect when interacting with the group repo; the expectation is that queries to the group repo will do what is required to consult any member repositories and fulfill client requests transparently.