[search] Introduce cubing-private exports for search.

This allows us to use functions across sub-packages without exporting them publicly. This is generally a sign of bad design, but it allows us to ship certain functionality without exposing an experimental API that we do not intend to support in the future.

In particular, this PR removes `[experimental]randomScrambleForEvent()` from `cubing/search`, since it's only meant to be used from `cubing/scramble`.

script/test/import-restrictions/main.js

@@ -10,6 +10,8 @@ const TARGET_INFOS_PATH = resolve(
   "./target-infos.js",
 );
 
+const CUBING_PRIVATE_SUFFIX = "/cubing-private";
+
 // Note that we have to use an extra `..` to back out of the file name
 const PATH_TO_SRC_CUBING = resolve(
   new URL(".", import.meta.url).pathname,
@@ -77,6 +79,11 @@ class Target {
           return undefined;
         }
 
+        // Allow cubing-private cross-package exports.
+        if (args.path.endsWith(CUBING_PRIVATE_SUFFIX)) {
+          args.path = args.path.slice(0, -CUBING_PRIVATE_SUFFIX.length);
+        }
+
         const resolved = resolve(args.resolveDir, args.path);
         if (this.dirPath === resolved) {
           this.checkImportable(args, forTarget);

src/cubing/scramble/index.ts

@@ -1 +1 @@
-export { experimentalRandomScrambleForEvent as randomScrambleForEvent } from "../search";
+export { randomScrambleForEvent } from "../search/cubing-private";

src/cubing/search/cubing-private/index.ts

@@ -0,0 +1 @@
+export { randomScrambleForEvent } from "../outside";

src/cubing/search/index.ts

@@ -1,5 +1,4 @@
 export {
-  randomScrambleForEvent as experimentalRandomScrambleForEvent,
   experimentalSolve3x3x3IgnoringCenters,
   experimentalSolve2x2x2,
   solveSkewb,