Refactor loading of modules - support remote modules with scheme

[mstoykov]

Previously remote modules were supported specifically without scheme.
With this change we specifically prefer if they are with a scheme. The
old variant is supported but prints a warning.
Other changes:

• If remote module requires a relative/absolute path that doesn't have a
  scheme it is relative/absolute given the remote module url.
• Support local files with file scheme.

fix #1037

[mstoykov]

Things that should probably be fixed:

• Some of the error messages: specifically when you try to load remote url and it fails make it seem like we tried loading from the fs and than the url and nothing worked which is lie, now and previously.

• The way k6 archive t.js works in case of https:// urls and probably file:// should be fixed. This is somewhat related to Running from archive still downloads scripts from the internet #887 .

• Make the loader(github/cdnjs) return the real URLs and put those in the archive ?!? related to the previous one

• Better test of when url.Parse fails.

[codecov]

Codecov Report

Merging #1046 into master will decrease coverage by 0.02%.
The diff coverage is 73.01%.

@@            Coverage Diff             @@
##           master    #1046      +/-   ##
==========================================
- Coverage   72.67%   72.65%   -0.03%     
==========================================
  Files         133      133              
  Lines        9846     9892      +46     
==========================================
+ Hits         7156     7187      +31     
- Misses       2274     2286      +12     
- Partials      416      419       +3

Impacted Files	Coverage Δ
loader/loader.go	81.59% <73.01%> (-8.28%)	⬇️

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[mstoykov]

superseded by #1059