Support relative pathing for `ng generate` statements

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

### Versions.
```@angular/cli: 1.0.4
node: 6.10.2
os: linux x64
@angular/common: 4.1.3
@angular/compiler: 4.1.3
@angular/core: 4.1.3
@angular/forms: 4.1.3
@angular/http: 4.1.3
@angular/platform-browser: 4.1.3
@angular/platform-browser-dynamic: 4.1.3
@angular/router: 4.1.3
@angular/cli: 1.0.4
@angular/compiler-cli: 4.1.3
```

### Repro steps.

From project root:
```
ng g component ./src/app/components/test
```

### The log given by the failure.
```
installing component
create src/app/src/app/components/test/test.component.scss
create src/app/src/app/components/test/test.component.html
create src/app/src/app/components/test/test.component.spec.ts
create src/app/src/app/components/test/test.component.ts
```

### Desired functionality.

Doesn't create src/app under the (already existing) src/app directory.

This is a minor change, but would then allow me to use autocomplete with the generator, even if the immediate fix is to just remove 'src/app' from the file location input before processing.

### Mention any other details that might be useful.

Reproduziere das Problem aus dem Projektstammverzeichnis mit `ng g component ./src/app/components/test` und untersuche, wie der Angular CLI-Generator diesen relativen Pfad auflöst. Als erledigt gilt die Aufgabe, wenn die generierten Dateien im vorhandenen Verzeichnis `src/app` abgelegt werden, ohne einen doppelten Pfad `src/app/src/app` zu erstellen.

Vom Indexierungsmodell aus dem Issue-Text verfasst.