Artifact Validator: don't throw away pages that have been requested as entries

[jpommerening]

The artifact_validator currently drops all pages that are not referenced from a flow.
This makes a lot of sense, as long as one is bundling a whole app. If you're trying to bundle just a part of your app to leverage webpack's code splitting, you'll be surprised that what you requested is missing from the artifacts bundle.

To reproduce using laxar-loader:

import artifacts from 'laxar-loader/artifacts?page=my-page';

Array.isArray( artifacts.pages ); // true
artifacts.pages.length === 1; // false, pages is empty

[jpommerening]

Fixed in v2.0.3