Remove most AssetNode subtypes.

[davidmorgan]

For #3811.

Remove subtypes that don't add any fields.

Improve naming in a few places, at least I hope these are clearer :) suggestions welcome.

syntheticSource -> missingSource
isReadable -> isFile
isInteresting -> changesRequireRebuild
isValidInput -> isTrackedInput

Improved docs in a few places.

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Package	Changed Files

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Changelog Entry ❗

Package	Changed Files
package:build_runner	build_runner/lib/src/watcher/change_filter.dart
package:build_runner_core	build_runner_core/lib/src/asset/finalized_reader.dart build_runner_core/lib/src/asset_graph/graph.dart build_runner_core/lib/src/asset_graph/node.dart build_runner_core/lib/src/asset_graph/serialization.dart build_runner_core/lib/src/generate/build_definition.dart build_runner_core/lib/src/generate/build_impl.dart build_runner_core/lib/src/generate/finalized_assets_view.dart build_runner_core/lib/src/generate/single_step_reader_writer.dart

Changes to files need to be accounted for in their respective changelogs.

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