refactor(router): organize deprecated loadChildren code (#47586)

This commit breaks out the code in @angular/router needed to support the
deprecated `loadChildren` signature into a separate file, which reduces the
opportunity for conflicts when patching that behavior in g3.

PR Close #47586

goldens/public-api/router/index.md

@@ -356,7 +356,7 @@ export interface IsActiveMatchOptions {
 }
 
 // @public
-export type LoadChildren = LoadChildrenCallback;
+export type LoadChildren = LoadChildrenCallback | ɵDeprecatedLoadChildren;
 
 // @public
 export type LoadChildrenCallback = () => Type<any> | NgModuleFactory<any> | Routes | Observable<Type<any> | Routes> | Promise<NgModuleFactory<any> | Type<any> | Routes>;

packages/router/src/deprecated_load_children.ts

@@ -0,0 +1,26 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import {Injector, NgModuleFactory} from '@angular/core';
+import {Observable} from 'rxjs';
+
+// This file exists to support the legacy `loadChildren: string` behavior being patched back into
+// Angular.
+
+/**
+ * Deprecated `loadChildren` value types.
+ *
+ * @publicApi
+ * @deprecated represents the deprecated type side of `LoadChildren`.
+ */
+export type DeprecatedLoadChildren = never;
+
+export function deprecatedLoadChildrenString(
+    injector: Injector, loadChildren: unknown): Observable<NgModuleFactory<any>>|null {
+  return null;
+}

packages/router/src/models.ts

@@ -9,6 +9,7 @@
 import {EnvironmentInjector, ImportedNgModuleProviders, InjectionToken, NgModuleFactory, Provider, ProviderToken, Type} from '@angular/core';
 import {Observable} from 'rxjs';
 
+import {DeprecatedLoadChildren} from './deprecated_load_children';
 import {ActivatedRouteSnapshot, RouterStateSnapshot} from './router_state';
 import {UrlSegment, UrlSegmentGroup, UrlTree} from './url_tree';
 
@@ -125,7 +126,7 @@ export type LoadChildrenCallback = () => Type<any>|NgModuleFactory<any>|Routes|
  * @see `LoadChildrenCallback`
  * @publicApi
  */
-export type LoadChildren = LoadChildrenCallback;
+export type LoadChildren = LoadChildrenCallback|DeprecatedLoadChildren;
 
 /**
  *

packages/router/src/private_export.ts

@@ -8,6 +8,7 @@
 
 
 export {ɵEmptyOutletComponent} from './components/empty_outlet';
+export {DeprecatedLoadChildren as ɵDeprecatedLoadChildren} from './deprecated_load_children';
 export {withPreloading as ɵwithPreloading} from './provide_router';
 export {assignExtraOptionsToRouter as ɵassignExtraOptionsToRouter, RestoredState as ɵRestoredState} from './router';
 export {ROUTER_PROVIDERS as ɵROUTER_PROVIDERS} from './router_module';

packages/router/src/router_config_loader.ts

@@ -10,7 +10,8 @@ import {Compiler, EnvironmentInjector, Injectable, InjectFlags, InjectionToken,
 import {ConnectableObservable, from, Observable, of, Subject} from 'rxjs';
 import {catchError, finalize, map, mergeMap, refCount, tap} from 'rxjs/operators';
 
-import {LoadChildren, LoadedRouterConfig, Route, Routes} from './models';
+import {deprecatedLoadChildrenString} from './deprecated_load_children';
+import {LoadChildren, LoadChildrenCallback, LoadedRouterConfig, Route, Routes} from './models';
 import {flatten, wrapIntoObservable} from './utils/collection';
 import {assertStandalone, standardizeConfig, validateConfig} from './utils/config';
 
@@ -122,7 +123,12 @@ export class RouterConfigLoader {
 
   private loadModuleFactoryOrRoutes(loadChildren: LoadChildren):
       Observable<NgModuleFactory<any>|Routes> {
-    return wrapIntoObservable(loadChildren()).pipe(mergeMap((t) => {
+    const deprecatedResult = deprecatedLoadChildrenString(this.injector, loadChildren);
+    if (deprecatedResult) {
+      return deprecatedResult;
+    }
+
+    return wrapIntoObservable((loadChildren as LoadChildrenCallback)()).pipe(mergeMap((t) => {
       if (t instanceof NgModuleFactory || Array.isArray(t)) {
         return of(t);
       } else {