java,runfiles: runfiles library in @bazel_tools

Also make most targets in `//src/tools/runfiles`
private. The user should depend on
`@bazel_tools//tools/runfiles:$LANG-runfiles`
instead.

See https://github.com/bazelbuild/bazel/issues/4460

RELNOTES[NEW]: java,runfiles: You can now depend on `@bazel_tools//tools/runfiles:java-runfiles` to get a platform-independent runfiles library for Java. See JavaDoc of https://github.com/bazelbuild/bazel/blob/master/src/tools/runfiles/java/com/google/devtools/build/runfiles/Runfiles.java for usage information.

Change-Id: Iba9113453222ae74ce42a324272711f613104891
PiperOrigin-RevId: 182022851
diff --git a/tools/runfiles/BUILD.tools b/tools/runfiles/BUILD.tools
new file mode 100644
index 0000000..d928523
--- /dev/null
+++ b/tools/runfiles/BUILD.tools
@@ -0,0 +1,6 @@
+package(default_visibility = ["//visibility:public"])
+
+alias(
+    name = "java-runfiles",
+    actual = "//src/tools/runfiles/java/com/google/devtools/build/runfiles",
+)