Allow specifying download link for dependencies in a dependency object

[TheGlitch76]

You should have the ability to recommend a download link for users in dependency objects, so that users can quickly find + (manually) download mods they are missing.

This is different from automatic dependency downloading; it would just be something like a url field on a dependency object that can be any web page.

[Kneelawk]

I could see this implemented as a mod separate from quilt_loader that either: opens the webpage in an external browser and expects the player to download the correct file to the correct place; or opens the webpage in an embeded browser that can automatically download to the right place, while also checking hashes to make sure the player actually downloaded the correct file.

This is just one idea though.