Properly resolve mocked node modules without package.json defined

[thymikee]

Summary

Fixes #4439

While constructing ModuleMap, the getMockName fn resolves like this:

./src/__mocks__/my-module.js 			-> "my-module" (1)
./src/__mocks__/my-module/sth.js		-> "my-module/sth" (2)
./src/__mocks__/my-module/index.js		-> "my-module/index" (3)

However, the ModuleMap#getMockModule doesn't account for the third scenario, when the manually mocked module is required as my-module – it just gets "my-module" as a second param. To fix this, I've added a fallback check for */index to mimic node resolution algorithm.

Not sure if this is the right place to put this fix, open to suggestions.
Possible other place to update the resolution logic would be: https://github.com/facebook/jest/blob/2745e3e6cc196f191bfb8f33625e45ec4b4a1ded/packages/jest-resolve/src/index.js#L230

Test plan

Added an integration test

[SimenB]

Nice and neat! Mind adding a changelog entry?

[codecov-io]

Codecov Report

Merging #6232 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #6232   +/-   ##
=======================================
  Coverage   64.33%   64.33%           
=======================================
  Files         219      219           
  Lines        8460     8460           
  Branches        3        3           
=======================================
  Hits         5443     5443           
  Misses       3016     3016           
  Partials        1        1

Impacted Files	Coverage Δ
packages/jest-haste-map/src/module_map.js	87.5% <100%> (ø)	⬆️

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[SimenB]

My gut tells me to populate the mock module map correctly in the first place, rather than extending the lookup. That might be difficult though, meaning this pragmatic approach is good enough

[cpojer]

Yes I think I'd prefer fixing this in the place where we write the module map, rather than where we read it.

[thymikee]

So if there's a module or module/index available in __mocks__ we should only write one of these into ModuleMap?

[cpojer]

Right, let's try this one.

[github-actions]

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