`externalSchematics` should install packages locally when they are no present in the `node_modules`

从 @angular-devkit/schematics 中的 externalSchematic 函数以及 packages/angular/pwa/pwa/index.ts 中引用的调用开始。跟踪外部包在 node_modules 中不存在时如何解析，然后确定应如何将安装集成到该流程中。当不可用的外部 schematic 包被安装并且 schematic 成功执行时，即表示完成。

🚀 Feature request

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Description

when creating own schematics, they can make use of other schematics by calling the externalSchematic function provided by the @angular-devkit/schematics collection.

To call an external schematic, the function can be used like this:

options = {};
externalSchematic('my-schematic-package', 'my-schematic-name', options);

But to execute an external schematic package it has to be part of the node_modules. Internally the CLI uses the function for example for calling the PWA schematic. This won't be problematic as is already installed as part of the @angular/schematics package and in fact it's part of the node_modules.

Once you'll try to install an external schematic which isn't already available in the node_modules the schematic fails.

A workaround is to manually integrate an installation procedure first and then call the function like this:

export const installNpmPackage = (
  context: SchematicContext,
  packageName: string,
): Promise<void> => {
  return new Promise<void>((resolve) => {
    context.logger.info(
      `📦 Installing package '${packageName}' for external schematic setup...`,
    );
    const spawnOptions: SpawnOptions = { stdio: 'inherit' };
    spawn('npm', ['install', packageName], spawnOptions).on(
      'close',
      (code: number) => {
        if (code === 0) {
          context.logger.info(
            `✅ '${packageName}' package installed successfully`,
          );
          resolve();
        } else {
          const errorMessage = `❌ installation of '${packageName}' package failed`;
          context.logger.error(errorMessage);
          throw new Error();
        }
      },
    );
  });
};

const packageName = '@briebug/cypress-schematic';
await installNpmPackage(context, packageName);
return externalSchematic(packageName, 'ng-add', {
  removeProtractor: true,
  addCypressTestScripts: true,
});

Describe the solution you'd like

It would be great to integrate the installation process for a package that's not present in the node_modules in the externalSchematic function.

Describe alternatives you've considered