Avoid reaching into root crate for ocaml rust flags

Summary:
This will no longer be possible with external cells. This diff
introduces slight duplication, but landing it first will unbreak ocamlrep
builds immediately and avoid a shipit desync.

Reviewed By: bigfootjon

Differential Revision: D64847706

fbshipit-source-id: 08bd28b3080250ce07f13984a9fa796d148a7532

common/ocaml/interop/defs.bzl

@@ -5,6 +5,9 @@
 # License, Version 2.0 found in the LICENSE-APACHE file in the root directory
 # of this source tree.
 
-load("@root//defs.bzl", _RUST_FLAGS_2018 = "RUST_FLAGS_2018")
-
-RUST_FLAGS_2018 = _RUST_FLAGS_2018
+# Try to keep in sync with all the client projects, like hack
+RUST_FLAGS_2018 = [
+    "-Drust-2018-idioms",
+    "-Dwarnings",
+    "-Dunused-crate-dependencies",
+]