fix(@ngtools/webpack): fix aot builds using npm packages that have lazy loaded modules

Currently, code splitting derived from routes with the loadChildren property only works
if the resolved source file is a *.ts file in the source directory of your angular-cli
project. If your project uses an npm package that has a route with loadChildren, the
build will resolve the lazy loaded module as a *.d.ts definition file and then attempt
to find an *.d.ts.ngfactory.ts file in the generated AOT directory, which does not exist.
This fixes the path generation logic so the build will look for a *.ngfactory.ts file
in $$_gendir/node_modules.

Author: maleetz

packages/@ngtools/webpack/src/plugin.ts

@@ -461,7 +461,8 @@ export class AotPlugin implements Tapable {
             if (this.skipCodeGeneration) {
               this._lazyRoutes[k] = lazyRoute;
             } else {
-              const lr = path.relative(this.basePath, lazyRoute.replace(/\.ts$/, '.ngfactory.ts'));
+              const factoryPath = lazyRoute.replace(/(\.d)?\.ts$/, '.ngfactory.ts');
+              const lr = path.join(path.sep, path.relative(this.basePath, factoryPath));
               this._lazyRoutes[k + '.ngfactory'] = path.join(this.genDir, lr);
             }
           });