Refactor file outputs to BuildOutputReader

[davidmorgan]

Combine three related classes, FinalizedReader, OptionalOutputTracker and FinalizedAssetsView, into BuildOutputReader.

Make it an output of the build, in BuildResult, instead of passed around separately.

Use the build output from the build result in the daemon and serve commands.

[github-actions]

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package:build_runner

build_runner/lib/src/build/build.dart build_runner/lib/src/build/build_result.dart build_runner/lib/src/build/build_series.dart build_runner/lib/src/build/finalized_assets_view.dart build_runner/lib/src/build_plan/testing_overrides.dart build_runner/lib/src/commands/daemon/asset_server.dart build_runner/lib/src/commands/daemon/daemon_builder.dart build_runner/lib/src/commands/serve/server.dart build_runner/lib/src/commands/serve_command.dart build_runner/lib/src/commands/watch/watcher.dart build_runner/lib/src/commands/watch_command.dart build_runner/lib/src/io/finalized_reader.dart

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