Add option to define a custom resolver

[dlmr]

Summary

This PR addresses #2837, implementing a new configuration option called resolver that can be used to implement a custom resolver.

This is a non breaking change.

Feedback about the approach is greatly appreciated and if this is a good way to solve #2837 in a general way. This works great for my use case.

Test plan

All existing tests pass along with the new one added for testing the new functionality.

[PAkerstrand]

Any updates for this? We need this type of functionality with linked dependencies during development...

[Phoenixmatrix]

I still don't know how Jest works internally that well. The resolver in this PR, does it take the original arguments and return a path to the new module, or does it return an actual module? (I'm having trouble telling since your mock returns a string, so it could be either).

For my use case, I need to be able to resolve paths to actual modules (the object itself), not just a path since the module could be in a totally different format (eg: AMD or even something custom), so CommonJS would not be able to load it.

[dlmr]

This resolver is expected to return a path to where the module is located, it does not allow you to return the actual module.

If we need to also make it possible to directly return the module we need to do a larger change somewhere else, not in jest-resolve. I'm unsure where that change would need to be performed but my guess right now is jest-runtime.

Both of the dependencies that @cpojer mentions in #2837, jest-resolve-dependencies and jest-resolve, operate on paths.

[dlmr]

@Phoenixmatrix Looked at it a little more now and it seems that jest-runtime is doing the actual loading of the modules. jest-runtime/src/index.js#L273-L294

It seems that this is already possible to replace with an existing configuration option, moduleLoader. jest-cli/src/runTest.js#L30

[codecov-io]

Codecov Report

Merging #2998 into master will decrease coverage by -0.01%.
The diff coverage is 28.57%.

@@            Coverage Diff             @@
##           master    #2998      +/-   ##
==========================================
- Coverage   67.86%   67.86%   -0.01%     
==========================================
  Files         147      148       +1     
  Lines        5349     5352       +3     
==========================================
+ Hits         3630     3632       +2     
- Misses       1719     1720       +1

Impacted Files	Coverage Δ
packages/jest-runtime/src/index.js	85.02% <ø> (ø)	✅
packages/jest-config/src/validConfig.js	100% <ø> (ø)	✅
packages/jest-config/src/normalize.js	85.9% <ø> (ø)	✅
packages/jest-resolve/src/defaultResolver.js	0% <0%> (ø)
packages/jest-resolve/src/index.js	16.16% <100%> (+2.3%)	✅

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[dlmr]

This PR is somewhat related to #2925 since it affects the use of resolve and make Jest less dependent on it.

[github-actions]

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