Build stages should not be included as a link

[rcjsuen]

FROM alpine AS base
FROM base

import { DockerfileLanguageServiceFactory } from 'dockerfile-language-service';
const service = DockerfileLanguageServiceFactory.createLanguageService();
let items = service.computeLinks("FROM alpine AS base\nFROM base");
console.log(items);

[ { range: { start: [Object], end: [Object] }, data: '_/alpine/' },
  { range: { start: [Object], end: [Object] }, data: '_/base/' } ]

Only the first line should be a valid link to Docker Hub. We shouldn't be returning two items here.