ng add fails to find app.module.ts when using custom tsconfig.json paths

### Bug Report or Feature Request (mark with an `x`)
```
- [X ] bug report -> please search issues before submitting
- [X ] feature request
```
Whether this is a bug or a feature depends on whether or not custom `paths` in `tsconfig.json` are supported with the Cli.

### Command (mark with an `x`)
```
- [X ] add
```

### Versions
Angular CLI: 7.0.2
Node: 10.8.0
OS: win32 x64
Angular: 7.0.0
... animations, common, compiler, compiler-cli, core, forms
... http, language-service, platform-browser
... platform-browser-dynamic, router

### Repro steps
- `ng new app-name` (pick defaults)
- Add `"paths": { "@app/*": ["src/app/*"] }` to `tsconfig.json`
- Update `main.ts` to use this new path
`import { AppModule } from '@app/app.module';`
`import { environment } from './environments/environment';`
- `ng add @angular/material`
- Select `Yes` for `? Set up browser animations for Angular Material?`

Repository all set up to reproduce this error is available [here](https://github.com/seangwright/ng-cli-add-error-with-tsconfig-paths).
Just clone and type `ng add @angular/material`.

### The log given by the failure
`Could not read Angular module file: /src/@app/app.module.ts`

### Desired functionality
Cli handles custom paths in `tsconfig.json`.

### Mention any other details that might be useful
Here is where material tries to get the `app.module.ts` path
https://github.com/angular/material2/blob/985774a4eaa14d1dcbf1ad96ab176043d38f433e/src/lib/schematics/ng-add/setup-project.ts#L56

Here is where the path to `app.module.ts` is being calculated in the schematics code
https://github.com/angular/angular-cli/blob/82f2bda2f59e4665611d1a75b51a62bae21ac340/packages/schematics/angular/utility/ng-ast-utils.ts#L78

I believe the above link is what is causing the problem.

I've always preferred absolute urls in Angular apps compared to the relative paths (which become very difficult to read for nested components).

Adding custom `paths` to `tsconfig.json` works pretty well to solve this problem. VSCode works fine with it and the Cli has no other issues I've run into.

If using `paths` isn't officially supported does the Angular team recommend using relative paths like `import { MyApiService } from '../../../../../my-api.service'`?

Start with packages/schematics/angular/utility/ng-ast-utils.ts, then compare how material2's setup-project.ts consumes the calculated app.module.ts path. Reproduce the failure with the linked repository by running ng add @angular/material after configuring tsconfig.json paths. Done means the command locates the module successfully when main.ts imports it through @app/app.module.

Written by the indexing model from the issue text.