feat(java_indexer): emit refs to JVM graph for externally defined nodes (#3800)

Experimental feature to support cross-language references for JVM-based
languages.  Currently, the indexer will be highly conservative when
deciding whether to emit references to the JVM language vs Java-native
nodes.  Only nodes within the same source file as the enclosing class
and JDK symbols will be considered Java-native when
--emit_jvm_references is enabled.

See: https://github.com/kythe/kythe/blob/master/kythe/java/com/google/devtools/kythe/analyzers/jvm/DESIGN.md#relations-with-higher-level-jvm-languages
6 files changed