Closes https://github.com/flutter/flutter/issues/148444 (code de-duplicated).
Closes https://github.com/flutter/flutter/issues/150877 (`--build-strategy=local`).
Closes https://github.com/flutter/flutter/issues/150884 (`--build-strategy=remote`).
Replaces duplicate code across ~3 commands (query, test, build) with a class called `BuildPlan`, which encapsulates (a) concurrency, (b) build configuration, (c) RBE, (d) LTO, and (e) build strategy. I also moved all of the validation of the build plan into `build_plan_test`, which gives us better coverage at a lower cost (less things to think about in that suite of tests).
I know the diff looks scary, but 1K of the 1.4K is tests.
/cc @gaaclarke @flar
173af3d95f