allow customized builder ordering by users

[jakemac53]

Fixes #3328

@explodus did you want to try this out?

You should be able to add a dependency override like this to try it (in your pubspec.yaml):

dependency_overrides:
  build_config:
    git:
      url: https://github.com/dart-lang/build.git
      ref: allow-customizing-builder-ordering
      path: build_config
  build_runner:
    git:
      url: https://github.com/dart-lang/build.git
      ref: allow-customizing-builder-ordering
      path: build_runner

And then configure it like this in your build.yaml:

global_options:
  source_gen:combining_builder:
    runs_before:
      - reflectable

[jakemac53]

cc @natebosch I added some validation of builder keys (when I was manually testing this feature I mistyped some... lol). I also prepped this for immediate release given that we have no other features planned.