Support --experimental_java_classpath=bazel

[cushon]

See #16426 (comment)

Originally the reduced classpath reduction happened during action execution, so changes in inputs would cause invalidation even if those inputs were pruned off the reduced classpath. But the classpath reduction now happens in Bazel, so in the example if libZ changes the action will still be a cache hit and won't be reexecuted:

https://cs.github.com/bazelbuild/bazel/blob/60ded2b53d4d9180c213e9e44a8c640368e60822/src/main/java/com/google/devtools/build/lib/rules/java/JavaCompileAction.java#L244

That optimization was implemented for Blaze, but parts of it are not wired up in Bazel, and attempting to enable it results in an NPE.

With Bazel built at 0dcfe21:

$ bazel build --experimental_java_classpath=bazel //src:bazel

FATAL: bazel crashed due to an internal error. Printing stack trace:
java.lang.RuntimeException: Unrecoverable error while evaluating node 'ActionLookupData{actionLookupKey=ConfiguredTargetKey{label=//src/main/java/com/google/devtools/build/lib/analysis/starlark/annotations/processor:annotation_preprocessor_lib, config=BuildConfigurationKey[a8f24905613097a24a99584d488eea2b48312ca68542cff50751864e342cea1f]}, actionIndex=0}' (requested by nodes 'ArtifactNestedSetKey{rawChildren=[File:[[<execution_root>]bazel-out/k8-opt-exec-EDC14992/bin]src/main/java/com/google/devtools/build/lib/analysis/starlark/annotations/processor/libannotation_preprocessor_lib.jar, File:[[<execution_root>]bazel-out/k8-opt-exec-EDC14992/bin]src/main/java/com/google/devtools/build/lib/analysis/starlark/annotations/libannotations_internal.jar]}')
        at com.google.devtools.build.skyframe.AbstractParallelEvaluator$Evaluate.run(AbstractParallelEvaluator.java:642)
        at com.google.devtools.build.lib.concurrent.AbstractQueueVisitor$WrappedRunnable.run(AbstractQueueVisitor.java:382)
        at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
        at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
        at java.base/java.lang.Thread.run(Unknown Source)
Caused by: java.lang.NullPointerException: Cannot invoke "com.google.devtools.build.lib.rules.java.JavaCompileActionContext.getDependencies(com.google.devtools.build.lib.actions.Artifact, com.google.devtools.build.lib.actions.ActionExecutionContext)" because "context" is null
        at com.google.devtools.build.lib.rules.java.JavaCompileAction.getReducedClasspath(JavaCompileAction.java:259)
        at com.google.devtools.build.lib.rules.java.JavaCompileAction.execute(JavaCompileAction.java:408)
        at com.google.devtools.build.lib.skyframe.SkyframeActionExecutor$ActionRunner.executeAction(SkyframeActionExecutor.java:1045)
        at com.google.devtools.build.lib.skyframe.SkyframeActionExecutor$ActionRunner.run(SkyframeActionExecutor.java:966)
        at com.google.devtools.build.lib.skyframe.ActionExecutionState.runStateMachine(ActionExecutionState.java:160)
        at com.google.devtools.build.lib.skyframe.ActionExecutionState.getResultOrDependOnFuture(ActionExecutionState.java:93)
        at com.google.devtools.build.lib.skyframe.SkyframeActionExecutor.executeAction(SkyframeActionExecutor.java:497)
        at com.google.devtools.build.lib.skyframe.ActionExecutionFunction.checkCacheAndExecuteIfNeeded(ActionExecutionFunction.java:826)
        at com.google.devtools.build.lib.skyframe.ActionExecutionFunction.computeInternal(ActionExecutionFunction.java:323)
        at com.google.devtools.build.lib.skyframe.ActionExecutionFunction.compute(ActionExecutionFunction.java:161)
        at com.google.devtools.build.skyframe.AbstractParallelEvaluator$Evaluate.run(AbstractParallelEvaluator.java:571)
        ... 4 more

This bug tracks making --experimental_java_classpath=bazel work, to improve performance of incremental Java builds.

[cushon]

This is a duplicate of #16635