Allow builder authors to define custom "environment checks"

[natebosch]

These would run before a build (maybe not every time though?) and would give a hook to check on the user's environment and offer help.

Some examples:

1. built_value could check that built_value_generator is in the dependencies so that it's builder will run.
2. A proto compiler could check that the executable is installed an in $PATH and either give a nice error message or offer to install the tools.

We'd want these to be isolated from the rest of the build.

[matanlurey]

I really like this idea, sort of a "doctor"-plugin.

[natebosch]

Bonus points if we can use the generic hooks to set up things for build_runner itself, like validation on the build.yaml file for #885

[jakemac53]

Imo anything opting into this should be able to return a success/failure state in some form as well so we don't attempt a build if things aren't set up properly. Not sure if we would need a way of overriding that check or not.

[jakemac53]

Also we talked offline but this should run in a separate isolate so that builders can't do bad things - that allows us to expose information to them in this phase that wouldn't otherwise be accessible (such as the package graph), without worrying about breaking hermetic builds.

[natebosch]

We might also want to consider only running this with a special command pub run build_runner doctor. We could also maybe do it automatically before some builds like the first build after .packages changes.

[matanlurey]

I'd love to see it happen when the build script would change.