Drop support for schemeless urls

[mstoykov]

Since #1059 it has been allowed to import URLs with a scheme or more specifically with https(file is also supported for local files).

Previous to this, it was specifically required that there is no scheme and an https:// was added to the front and tried. For backward compatibility, this was left and a message was added that this is deprecated and instead https:// should be prepended.

IMO it is time for this to be dropped, both for simplifying the code and the error message (which was revised in #1439), but also to make it more distinguished between remote URLs and short URLs for github and cdnjs which still won't need https:// as they are basically not ... correct URLs, in the sense that they don't actually point to what is downloaded when k6 "resolves" them.

[na--]

To be clear, we are not dropping the support for "magic" URLs (e.g. k6 run github.com/loadimpact/k6/samples/http_get.js), right? Given how we've used them in a ton of official docs examples over the years, I think we should just heavily discourage them and remove them from the docs for now... 😞

[mstoykov]

Yes,

make it more distinguished between remote URLs

was supposed to confirm that we are leaving them and now they will be less likely to be confused with "real" URLs

[mstoykov]

Actually this was duplicated and then fixed in #3287 and #3451