Add MODULE.bazel and //stardoc/private to distro tarball (#154)

Needed for a bzlmod-compatible release
diff --git a/BUILD b/BUILD
index ef68dea..a9859d7 100644
--- a/BUILD
+++ b/BUILD
@@ -31,7 +31,9 @@
         "CHANGELOG.md",
         "CONTRIBUTORS",
         "LICENSE",
+        "MODULE.bazel",
         "//stardoc:distro_srcs",
+        "//stardoc/private:distro_srcs",
         "//stardoc/proto:distro_srcs",
     ] + glob(["*.bzl"]),
     visibility = ["//:__subpackages__"],
diff --git a/stardoc/private/BUILD b/stardoc/private/BUILD
index c0adf6d..e3561b1 100644
--- a/stardoc/private/BUILD
+++ b/stardoc/private/BUILD
@@ -5,3 +5,14 @@
     srcs = ["stardoc.bzl"],
     visibility = ["//stardoc:__pkg__"],
 )
+
+# Sources needed for release tarball.
+filegroup(
+    name = "distro_srcs",
+    srcs = [
+        "BUILD",
+    ] + glob([
+        "*.bzl",
+    ]),
+    visibility = ["//:__pkg__"],
+)