Remove magic resolution of modules imports from CDNJS/GitHub

[imiric]

We've seen intermittent failures of TestCDNJS because their API sometimes returns empty results (files is empty). Right now you can see this if you request https://api.cdnjs.com/libraries/Faker a few times.

To avoid test failures we should skip this test for now, but this issue can also serve as a starting point for discussing eventually removing the "magic" URL loading from CDNJS or maybe even GitHub, to avoid confusing users (e.g. https://github.com/loadimpact/k6-docs/pull/8).

### Tasks
- [ ] Warning about usage/future removal
- [ ] Remove in v0.53

[mstoykov]

as I mentioned in the internal discussion, I am for disabling both github and cdnjs magic urls and the schemeless remote ones as well, but add a flag to enable them (I would argue we don't need separate flags). Have it this way for ... a year and drop them altogether. Obviously we will need to also remove them from the documentation and make certain all our *-to-k6 tools generate only scripts not using both. We will probably also have some logic that if you run an archive created from before the version we release that the flag is ON instead of OFF but we still print a warning ...

[mstoykov]

I have update the issue to reflect the current consensus of going forward with removing this magical resolution.

For v0.51.0 we will be adding a warning. Anc currently plan to drop it in 0.53.0