Refactor to expose dependencies on NodeContext and avoid NodeContextRepository lookups

[dfederm]

NodeContext contains the ProjectGraphNode and a NodeContextRepository would be used to look up the NodeContext objects for dependencies. This is deceptively expensive since the lookup requires creating a NodeDescriptor, which allocates a SortedDictionary and copies the global properties to it.

This refactor has NodeContext contain a list of its dependencies (in NodeContext form already) to avoid all these lookups. This actually cascades to quite a bit of simplification and removal of some plumbing to the cache client layer since it was also iterating dependencies so required the NodeContextRepository.