Handle build config changes without restarting.

[davidmorgan]

Now that build config is loaded at runtime and not part of the generated script, it can be reloaded without restarting.

Remove log output about build_runner internal steps: these are very quick, it doesn't add much.

Remove log output about whether it's an incremental build or if not, why. You can tell if it's an incremental build from the detailed output about what's being built. Some tests were relying on this output for verification: change to better test cases that produce different output on disk instead.

Improvements to BuildRunnerTester.expectNoOutput for when it fails.

[github-actions]

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