[BEAM-4135] Fork the DirectRunner Engine

[tgroh]

For development velocity, the DirectRunner's two implementations can share a
large number of underlying implementations, but due to limitations on intermediate
data type representations and the depth of the dependency chains, partial migrations
end with extremely messy generic types. Forking the two implementations in order
to minimize the need to modify the Java-SDK specific implementations of the DirectRunner
is likely to significantly simplify initial development efforts, with the ability to make
future components which maintain a high degree of complexity generic and shared between
the two processing implementations.

This change is a fork + prune, removing as many classes as seems sensible. Follow-up
changes will migrate Java SDK types (AppliedPTransform and PCollection being the two
primary types) to the corresponding portable representations (PTransformNode and PCollectionNode).

Some of the forked utilities (e.g. WatermarkManager) should be deduplicated,
as they are sufficiently generic at their current state. This will occur in a follow-up change.

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[tgroh]

R: @youngoli

[youngoli]

Reviewed changes between forked files and non-forked files. Some files had further changes than just forking, which I'll sum up here for anyone else reading this:

DirectTransformExecutor.java - Removes usages of ModelEnforcement.
EvaluationContext.java - Changes usages of DirectGraph to ExecutableGraph. Removes side input code.
ExecutorServiceParallelExecutor.java - Changes usages of DirectGraph to ExecutableGraph. Removes usages of ModelEnforcement.
PipelineExecutor.java - Changes usages of DirectGraph to ExecutableGraph.
QuiescenceDriver.java - Changes usages of DirectGraph to ExecutableGraph.
RootProviderRegistry.java - Removes the function for returning a native Java registry.
TransformEvaluatorRegistry.java - Removes code used for creating a native Java SDK registry.
WatermarkManager.java - Changes usages of DirectGraph to ExecutableGraph.

All the changes look good to me.

[youngoli]

Minor nit: Two imports on same line.