Add unit tests for build process

[buenos-nachos]

I think this might be the last major piece of functionality that needs tests. Though to be honest, the title is more like "Make the build process code more unit-testable, and then make tests for it"

What we need

• Split up the build process functionality into smaller, more testable pieces
• As each new piece is created, add tests to ensure that any refactoring doesn't cause existing functionality to break

[buenos-nachos]

Quick update on this: doing this took a little bit more time than I thought (as you can see by this turning into three separate, hefty PRs)

The modules PR is almost done; Ben and I are going to get together later today to make a few additions to the functionality before merging in

I should also be able to wrap up the templates PR today. The code is split off and refactored, but still needs the tests added. It's going to follow a similar approach to the modules PR, so I just wanted to make sure that the modules PR made sense before doing the same thing for the templates