Explicitly export files needed by other packages

Add an explicit export statement (with the same visibility) for
files that are currently only implicitly exported and used by other
pacakges. This prepares changing the visibility of implicitly-exported
files to be package private without causing a breakage.

PiperOrigin-RevId: 280171386
Change-Id: I083a9d4f82067b4d4c94d7b554013aeb91e401cc
diff --git a/tools/migration/BUILD b/tools/migration/BUILD
index da0af7e..2ddd447 100644
--- a/tools/migration/BUILD
+++ b/tools/migration/BUILD
@@ -129,3 +129,5 @@
         "ctoolchain_compare.bzl",
     ],
 )
+
+exports_files(["ctoolchain_compare.bzl"])