Expose GetNodeFor* method on the project graph

Currently we only expose entry points and an unordered list of nodes.

It would be convenient to allow API consumers to fetch a node by project file. One wrinkle is that a project make actually have multiple nodes if global properties are different. We'll need to think about how the API looks.

Idea:
```
// Common case is no global props, so make that convenient. This can likely be a O(1) lookup.
ProjectGraphNode GetNodeForProject(string projectFile, IDictionary globalProperties = null)

// Gets all nodes matching the project name. This will likely be an O(n) search.
ProjectGraphNode[] GetNodesForProject(string projectFile)
```