Add a script to compute minimal roots for check_deps. (#932)

This avoids needing to have nearly as many rules here which should
reduce its churn.

I've tested that this reaches the exact same set of transitive
dependencies.

Note, only the last commit here is new.

executable_semantics/ast/BUILD

@@ -2,10 +2,7 @@
 # Exceptions. See /LICENSE for license information.
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
-package(default_visibility = [
-    "//bazel/check_deps:__pkg__",
-    "//executable_semantics:__subpackages__",
-])
+package(default_visibility = ["//executable_semantics:__subpackages__"])
 
 cc_library(
     name = "ast",