Decompose bootstrapper into separate classes

[rd4398]

While doing a spike on multiple version bootstrap, it was discovered that decomposing the bootstrap class into separate classes will act as a good first step to implement #878.

@dhellmann stated this in a PR review for the spike #909:

"Perhaps a good first step is to decompose the Bootstrapper class into separate classes for managing the resolution of requirements (including with the cache, the different ways to look up values using the requirement, git URL, etc.) from the management of the overall bootstrap process (including recursing for dependencies, performing builds, etc.). Extracting the resolver management into a separate class will make it easier to see where we are performing resolution in the bootstrapper, and where we might change the API of that new class to make it easy for the bootstrapper to loop over lists of results."