Expose "project_name" to contexts

[beckjake]

Describe the feature

We should expose the project name in dbt contexts (at least during runtime and parsing, which is easy). This is useful, for example, to check if a model is from the currently running project. It also would provide a nice way for users to access macros that might be hard to name: "the current project's macro, if it exists, or my dependency macro if it doesn't" - currently I don't know how to even express that in dbt!

Describe alternatives you've considered

Users can currently access the project name via gross hacks like ref.config.project_name. We should probably remove that access as part of this.
We could also fix macro namespacing to be a bit more accessible to users, but that is a very large task. This feature is much simpler.

Who will this benefit?

Mostly macro developers, I imagine.