Break eager cycle for "deprecate" function

There is a module cycle involving `@ember/debug/index` and `@ember/debug/lib/deprecate`, and it's somewhat intentional since both are sharing mutable state with the other that's intended to allow users to replace the implementations.

The cycle wouldn't be a big deal except for the fact that index **eagerly** consumes the `deprecate` function from `lib/deprecate`, which means that if you get unlucky in the evaluation order it just explodes.

There's no need to eagerly consume it. This PR makes the consumption lazy.

In general `export function` is resilient to cycles in a way that `export let` is not. There are a bunch more places in `@ember/debug` that are using `export let` when they should probably be using `export function` instead, especially because these function are supposed to *always* be present (even in prod they are no-ops, not missing).

Author: ef4

packages/@ember/debug/index.ts

@@ -2,7 +2,7 @@ import { isChrome, isFirefox } from '@ember/-internals/browser-environment';
 import type { AnyFn } from '@ember/-internals/utility-types';
 import { DEBUG } from '@glimmer/env';
 import type { DeprecateFunc, DeprecationOptions } from './lib/deprecate';
-import _deprecate from './lib/deprecate';
+import defaultDeprecate from './lib/deprecate';
 import { isTesting } from './lib/testing';
 import type { WarnFunc } from './lib/warn';
 import _warn from './lib/warn';
@@ -75,7 +75,7 @@ let assert: AssertFunc = noop as unknown as AssertFunc;
 let info: InfoFunc = noop;
 let warn: WarnFunc = noop;
 let debug: DebugFunc = noop;
-let deprecate: DeprecateFunc = noop;
+let currentDeprecate: DeprecateFunc | undefined;
 let debugSeal: DebugSealFunc = noop;
 let debugFreeze: DebugFreezeFunc = noop;
 let runInDebug: RunInDebugFunc = noop;
@@ -86,6 +86,10 @@ let deprecateFunc: DeprecateFuncFunc = function () {
   return arguments[arguments.length - 1];
 };
 
+export function deprecate(...args: Parameters<DeprecateFunc>): ReturnType<DeprecateFunc> {
+  return (currentDeprecate ?? defaultDeprecate)(...args);
+}
+
 if (DEBUG) {
   setDebugFunction = function (type: DebugFunctionType, callback: Function) {
     switch (type) {
@@ -98,7 +102,7 @@ if (DEBUG) {
       case 'debug':
         return (debug = callback as DebugFunc);
       case 'deprecate':
-        return (deprecate = callback as DeprecateFunc);
+        return (currentDeprecate = callback as DeprecateFunc);
       case 'debugSeal':
         return (debugSeal = callback as DebugSealFunc);
       case 'debugFreeze':
@@ -314,8 +318,6 @@ if (DEBUG) {
     }
   });
 
-  setDebugFunction('deprecate', _deprecate);
-
   setDebugFunction('warn', _warn);
 }
 
@@ -353,7 +355,6 @@ export {
   info,
   warn,
   debug,
-  deprecate,
   debugSeal,
   debugFreeze,
   runInDebug,