python,runfiles: runfiles library in @bazel_tools

Also update the Python stub script template to set
$RUNFILES_MANIFEST_FILE or $RUNFILES_DIR so the
runfiles library only needs to look for those.

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

RELNOTES[NEW]: python,runfiles: You can now depend on `@bazel_tools//tools/runfiles:py-runfiles` to get a platform-independent runfiles library for Python. See DocString of https://github.com/bazelbuild/bazel/blob/master/src/tools/runfiles/runfiles.py for usage information.

Change-Id: I4f68a11cb59f2782e5203e39fe60cc66b46023a2
PiperOrigin-RevId: 184515490
diff --git a/src/create_embedded_tools.py b/src/create_embedded_tools.py
index 352069c..0bcbdef 100644
--- a/src/create_embedded_tools.py
+++ b/src/create_embedded_tools.py
@@ -44,6 +44,7 @@
      lambda x: 'tools/jdk/ExperimentalTestRunner_deploy.jar'),
     ('*Runner_deploy.jar', lambda x: 'tools/jdk/TestRunner_deploy.jar'),
     ('*singlejar', lambda x: 'tools/jdk/singlejar/singlejar'),
+    ('src/tools/runfiles/runfiles.py', lambda x: 'tools/runfiles/runfiles.py'),
     ('*launcher.exe', lambda x: 'tools/launcher/launcher.exe'),
     ('*def_parser.exe', lambda x: 'tools/def_parser/def_parser.exe'),
     ('*ijar.exe', lambda x: 'tools/jdk/ijar/ijar.exe'),