Enforce only changing certain fields in build.<config>.yaml files

[jakemac53]

Today it looks like we just rely on the user to keep these in sync, we should actually merge the two files and only allow you to overwrite the options fields for builders.

Otherwise in the best case you have to duplicate a lot between files, and in the worst case users end up invalidating their entire build any time they switch configurations.

[matanlurey]

Do we see any downsides in restricting in this much?

Adding a restriction that only options can be set is pretty easy, otherwise.

/cc @jakemac53 @natebosch

[natebosch]

I think the restriction makes sense, though it does point towards potentially splitting up the file #885

For now we can just be careful how this is documented and make sure we have helpful error messages.

[jakemac53]

Do we see any downsides in restricting in this much?

Not that I know of - if we allow deeper changes then it would require full rebuilds when people change their --config option (the build structure would change).