ng add @angular/pwa cannot find AppModule if imported through index.ts

### Bug Report or Feature Request (mark with an `x`)
```
- [x] bug report -> please search issues before submitting
- [ ] feature request
```

### Area
```
- [ ] devkit
- [x] schematics
```

### Versions

```
$ node --version
v8.11.2
```

```
$ npm --version
5.6.0
```

### Repro steps

* Create an Angular 6 application
* If not present, create a `index.ts` in the App module folder with at least the following content:
```typescript
export * from './app.module';
```
* In the `main.ts`, import the App module indirectly by using its parent folder:
```typescript
import './polyfills.ts';

import { platformBrowserDynamic } from '@angular/platform-browser-dynamic';
import { enableProdMode } from '@angular/core';
import { environment } from './environments/environment';
import { AppModule } from './app'; // works because ./app/index.ts exports AppModule

if (environment.production) {
enableProdMode();
}

platformBrowserDynamic().bootstrapModule(AppModule);
```
* Now use angular CLI to add PWA
```
$ ng add @angular/pwa --project
```

### The log given by the failure

```
Installing packages for tooling via npm.

+ @angular/pwa@0.6.8
updated 1 package in 11.487s
Installed packages for tooling via npm.
Could not read file (/src/app.ts).
```

### Desired functionality

Make PWA install work even when importing the AppModule indirectly (through an `index.ts`).

To be more accurate, the `findBootstrapModulePath` function from `ng-ast-utils.d.ts` should properly identify the AppModule path in this setup.
https://github.com/angular/angular-cli/blob/44086c60ff1d6c26d30850bef125120f6c498ac1/packages/schematics/angular/utility/ng-ast-utils.ts#L49

This method is called by `updateAppModule` from https://github.com/angular/angular-cli/blob/8ce5ef48b81af484c37123d36422b3c58ddc1ffd/packages/schematics/angular/service-worker/index.ts#L99

The failure occurs when trying to update the content of a file which does not exists. Checking the file exists and displaying a better message would also help.

### Mention any other details that might be useful

The issue was encountered when attempting to add PWA to our [NGXP Seed](https://github.com/Monogramm/ngxp-seed) web side.
```
$ git clone https://github.com/Monogramm/ngxp-seed.git
$ git checkout develop
$ ng add @angular/pwa --project web
```

Since we managed to find out the root cause, the workaround of using the complete path will be used for now.

Start in packages/schematics/angular/utility/ng-ast-utils.ts at findBootstrapModulePath, then follow its use in packages/schematics/angular/service-worker/index.ts by updateAppModule. Reproduce with an AppModule re-exported through app/index.ts and ng add @angular/pwa; done means the indirect import resolves to the real module file instead of attempting to update /src/app.ts.

Written by the indexing model from the issue text.