Lock module resolution after initialization

[mstoykov]

What?

Make it an error to try to resolve not previously resolved modules, after initialization

Why?

Partly because it should be given how we make k6 work and want k6 archive and k6 cloud to "just work".

Partly because this ended up being the only not completely hackish way to know that we are past the initialization state which was needed in order to make a warning only in the init context.

See #3681

Checklist

• I have performed a self-review of my code.
• I have added tests for my changes.
• I have run linter locally (make lint) and all checks pass.
• I have run tests locally (make tests) and all tests pass.
• I have commented on my code, particularly in hard-to-understand areas.

Related PR(s)/Issue(s)

[codebien]

The logic LGTM, just a request for improving the comment.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 81.81818% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 73.33%. Comparing base (a62140c) to head (ba1af07).
Report is 7 commits behind head on master.

❗ Current head ba1af07 differs from pull request most recent head a9da76a. Consider uploading reports for the commit a9da76a to get more accurate results

Files	Patch %	Lines
js/modules/resolution.go	60.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3680   +/-   ##
=======================================
  Coverage   73.33%   73.33%           
=======================================
  Files         278      278           
  Lines       20329    20338    +9     
=======================================
+ Hits        14908    14915    +7     
- Misses       4477     4479    +2     
  Partials      944      944           

Flag	Coverage Δ
macos	?
ubuntu	73.28% <81.81%> (+0.01%)	⬆️
windows	73.18% <81.81%> (+0.01%)	⬆️

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