Deprecations before build major version release.

[davidmorgan]

Deprecate things that look unused and will be removed shortly in 4.0.0:

• AssetWriterSpy is obsoleted by comprehensive testing support in build_test TestReaderWriter.
• MultiplexingBuilder looks like a remnant of a long-gone era; the only use of it on pub is in build_barback, barback was from before build.
• To match how BuildStep works, move PostProcessBuildStep implementation to build_runner_core, leaving just the API. Constructing a PostProcessBuildStep was package private (under src/, not exported), so this is non breaking.
• Move StageTracker to build_runner_core since it's only used in APIs that will move there. Re-export it for now.

[github-actions]

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