feat(compiler-cli): lower loadChildren fields to allow dynamic module paths (#23088)

Computing the value of loadChildren does not work externally, as the CLI
needs to be able to detect the paths referenced to properly set up
codesplitting. However, internally, different approaches to codesplitting
require hashed module IDs, and the computation of those hashes involves
something like:

{path: '...', loadChildren: hashFn('module')}

ngc should lower loadChildren into an exported constant in that case.

This will never break externally, because loadChildren is always a
string externally, and a string won't get lowered.

PR Close #23088

packages/compiler-cli/test/BUILD.bazel

@@ -69,6 +69,7 @@ jasmine_node_test(
         "//packages/common:npm_package",
         "//packages/core:npm_package",
         "//packages/platform-browser:npm_package",
+        "//packages/router:npm_package",
     ],
     deps = [
         ":ngc_lib",

packages/compiler-cli/test/ngc_spec.ts

@@ -869,6 +869,40 @@ describe('ngc transformer command-line', () => {
         expect(mymoduleSource).toMatch(/ɵ0 = .*'test'/);
       });
 
+      it('should lower loadChildren', () => {
+        write('mymodule.ts', `
+          import {Component, NgModule} from '@angular/core';
+          import {RouterModule} from '@angular/router';
+          
+          export function foo(): string {
+            console.log('side-effect');
+            return 'test';
+          }
+
+          @Component({
+            selector: 'route',
+            template: 'route',
+          })
+          export class Route {}
+
+          @NgModule({
+            declarations: [Route],
+            imports: [
+              RouterModule.forRoot([
+                {path: '', pathMatch: 'full', component: Route, loadChildren: foo()}
+              ]),
+            ]
+          })
+          export class MyModule {}
+        `);
+        expect(compile()).toEqual(0);
+
+        const mymodulejs = path.resolve(outDir, 'mymodule.js');
+        const mymoduleSource = fs.readFileSync(mymodulejs, 'utf8');
+        expect(mymoduleSource).toContain('loadChildren: ɵ0');
+        expect(mymoduleSource).toMatch(/ɵ0 = .*foo\(\)/);
+      });
+
       it('should be able to lower supported expressions', () => {
         writeConfig(`{
           "extends": "./tsconfig-base.json",