create a core package that has building blocks for CLI

[saihaj]

Idea is to create a code first interface so building functions/helpers that will be used by the CLI. This can be an API in future we can expose for anyone trying to extend functionality on their own.

Started with the idea of refactoring to use https://github.com/graphprotocol/graph-tooling/pull/1394 but soon ran into issues cause everything is very mingled together and it is not very easy to just refactor one piece.

So idea here is create function that we can call and function we can easily test. https://github.com/graphprotocol/graph-tooling/pull/1415