Add a leading underscore to build_defs.bzl file to indicate that it's supposed to be private to absl.

py2py3_test_binary mostly won't work in user's bazel repository anyway, since it relies on "//absl:py3_mode" & "//absl:py2_mode" defined in *user*'s repository.

PiperOrigin-RevId: 260035995
Change-Id: I3cd92e22aa3bcb7b5cd261993984aae90781cdc4
diff --git a/absl/BUILD b/absl/BUILD
index ea93b60..fbd89db 100644
--- a/absl/BUILD
+++ b/absl/BUILD
@@ -2,7 +2,7 @@
 
 exports_files(["LICENSE"])
 
-load(":build_defs.bzl", "py2py3_test_binary")
+load(":_build_defs.bzl", "py2py3_test_binary")
 
 py_library(
     name = "app",
diff --git a/absl/build_defs.bzl b/absl/_build_defs.bzl
similarity index 96%
rename from absl/build_defs.bzl
rename to absl/_build_defs.bzl
index edfb0fb..4e13abd 100644
--- a/absl/build_defs.bzl
+++ b/absl/_build_defs.bzl
@@ -1,4 +1,4 @@
-"""Helpers for absl build rules."""
+"""Private helpers for absl build rules."""
 
 def py2py3_test_binary(name, **kwargs):
     """Create the same binary with different python versions for testing.
diff --git a/absl/logging/BUILD b/absl/logging/BUILD
index 8f32874..16f8776 100644
--- a/absl/logging/BUILD
+++ b/absl/logging/BUILD
@@ -2,7 +2,7 @@
 
 exports_files(["LICENSE"])
 
-load("//absl:build_defs.bzl", "py2py3_test_binary")
+load("//absl:_build_defs.bzl", "py2py3_test_binary")
 
 py_library(
     name = "logging",
diff --git a/absl/testing/BUILD b/absl/testing/BUILD
index 493a49c..193e81a 100644
--- a/absl/testing/BUILD
+++ b/absl/testing/BUILD
@@ -2,7 +2,7 @@
 
 exports_files(["LICENSE"])
 
-load("//absl:build_defs.bzl", "py2py3_test_binary")
+load("//absl:_build_defs.bzl", "py2py3_test_binary")
 
 py_library(
     name = "absltest",