add platform_bzl lib

Author: aignas

python/private/pypi/BUILD.bazel

@@ -342,6 +342,11 @@ bzl_library(
     ],
 )
 
+bzl_library(
+    name = "platform_bzl",
+    srcs = ["platform.bzl"],
+)
+
 bzl_library(
     name = "pypi_repo_utils_bzl",
     srcs = ["pypi_repo_utils.bzl"],