fix: remove duplicate imports when migrating standalone components

[sean-perkins]

This PR resolves an issue where existing imports to components would be persisted when migrating a standalone component, resulting in duplicate import statements after the migration.

e.g.:

import { IonContent } from "@ionic/angular";

@Component({
  template: `<ion-content></ion-content>`
  standalone: true
})
export class MyComponent {
  @ViewChild(IonContent) content!: IonContent;
}

Resulted in:

import { IonContent } from "@ionic/angular/standalone";
import { IonContent } from "@ionic/angular/standalone";

@Component({
  template: `<ion-content></ion-content>`
  standalone: true,
  imports: [IonContent]
})
export class MyComponent {
  @ViewChild(IonContent) content!: IonContent;
}

This work is split from the reproductions provided in #13