Add java_sources to the depmap-info.

More context:
Ity added this diff long long ago https://rbcommons.com/s/twitter/r/733 to include java_sources in self.context which had some performance issues.

I went ahead and removed it https://rbcommons.com/s/twitter/r/1130.
We relied on java_sources to be already present in self.targets for depmap goal.

This diff fixes the issue where for a scala target owning java_sources, the java_target is included in depmap output under targets found.

Without this patch, the java_sources is mising from targets but is present in targets['examples/src/scala/com/pants/example/scala_with_java_sources:scala_with_java_source']['targets']

[tw-172-25-146-131 pants]$ ./pants goal depmap --depmap-project-info examples/src/scala/com/pants/example/scala_with_java_sources:
*** Running pants in dev mode from /Users/tdesai/projects/pants/src/python/pants/bin/pants_exe.py ***
{
    "libraries": {},
    "targets": {
        "examples/src/scala/com/pants/example/scala_with_java_sources:scala_with_java_sources": {
            "libraries": [
                "org.scala-lang:scala-library:2.9.3"
            ],
            "is_code_gen": false,
            "targets": [
                ":scala-library-2.9.3",
                "examples/src/java/com/pants/examples/java_sources:java_sources"
            ],
            "roots": [
                {
                    "source_root":   "/Users/tdesai/projects/pants/examples/src/scala/com/pants/example/scala_with_java_sources",
                    "package_prefix": "com.pants.example.scala_with_java_sources"
                }
            ],
            "target_type": "SOURCE"
        },

        "examples/src/java/com/pants/examples/java_sources:java_sources": {
            "libraries": [],
            "is_code_gen": false,
            "targets": [],
            "roots": [
                {
                    "source_root": "/Users/tdesai/projects/pants/examples/src/java/com/pants/examples/java_sources",
                    "package_prefix": "com.pants.examples.java_sources"
                }
            ],
            "target_type": "SOURCE"
        },
        ":scala-library-2.9.3": {
            "libraries": [],
            "is_code_gen": false,
            "targets": [],
            "roots": [],
            "target_type": "SOURCE"
        }
    }
}

Testing Done:
yes.

./pants tests/python/pants_test/tasks:depmap_integration

and
./pants tests/python/pants_test/tasks:depmap

pass

CI:
https://travis-ci.org/pantsbuild/pants/builds/40829185

Reviewed at https://rbcommons.com/s/twitter/r/1324/