Refine the dependency graph for build scripts

Refine the dependency graph for build scripts

Build scripts can immediately start building as soon as all build dependencies
are available and not need to wait for normal dependencies. This commit also
includes a number of refactorings and reorganizations to tidy up how build
scripts are processed.

One primary piece of state introduced in this commit is a shared Arc<Mutex<T>>
which contains information about the processed build scripts as compilation
continues. Compilation commands will draw information from this state and build
scripts will feed information back into this state to ensure it's up to date.