Support 'bazel mod tidy' (#64)

Add support for running `bazel mod tidy`.

multitool/extension.bzl

diff --git a/multitool/extension.bzl b/multitool/extension.bzl
index 8ab4090..e76994d 100644
--- a/multitool/extension.bzl
+++ b/multitool/extension.bzl
@@ -21,6 +21,12 @@
         module_ctx = module_ctx,
     )
 
+    return module_ctx.extension_metadata(
+        root_module_direct_deps = ["multitool"],
+        root_module_direct_dev_deps = [],
+        reproducible = True,  # repo state is only a function of the lockfile
+    )
+
 multitool = module_extension(
     implementation = _extension,
     tag_classes = {