Explicitly export def.bzl file for stardoc

Stardoc has a longstanding issue where you have to refer to an
entrypoint for it's scan and that *must* be a specific file.

go/BUILD.bazel

@@ -28,6 +28,10 @@ toolchain_type(
     visibility = ["//visibility:public"],
 )
 
+# Exported so the docs can live in another dir.
+# https://github.com/bazelbuild/stardoc/issues/57
+exports_files(["def.bzl"])
+
 bzl_library(
     name = "def",
     srcs = ["def.bzl"],