feat: support . in exports field

[SimenB]

Summary

If we don't have a main field, let's attempt to set it to the result of resolving . (i.e. "main" entry of package) if it exists. Since we do nothing unless main is already missing I think this is a safe thing to do and it's not a breaking change. Can revert and land for Jest 28 if it proves to be breaking, though.

Ref #9771 (comment)

Test plan

Tests added

[SimenB]

I think this is ready now.

One issue is that if the user provides a packageFilter this overrides it... I guess I can call any user provided filter first then ours

[SimenB]

we don't need this anymore as this just tests that the condition node, browser etc of the main entry point are supported, and since we add support to jest itself for the entry point (and the things we test don't have a main), the rest works.

[SimenB]

since we now wrap the function, it's not actually passed as is through, so I had to make some changes so packageFilter is actually called and assert on that

[SimenB]

Hmm, the change I had to make in the last commit worries me. This module used to work since there's a index.js file in root, but exports just has node and default (which is browser) conditions. We don't provide the node condition since it can mean require or import.

While this is probably fine in most cases, it might mean we must wait with this PR until Jest 28 as well...

Or at least we need to consider adding node condition as well, then passing the file we get back through the checks for whether it should be loaded as ESM or not and throw if it is.

EDIT: Some misunderstandings here - we should always check if file resolved from exports should be loaded as ESM or not, what condition it came from is irrelevant.

[codecov-commenter]

Codecov Report

Merging #11919 (5c97bf3) into main (b5aec03) will increase coverage by 0.02%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##             main   #11919      +/-   ##
==========================================
+ Coverage   68.72%   68.74%   +0.02%     
==========================================
  Files         323      323              
  Lines       16631    16649      +18     
  Branches     4799     4805       +6     
==========================================
+ Hits        11429    11445      +16     
- Misses       5169     5171       +2     
  Partials       33       33              

Impacted Files	Coverage Δ
packages/jest-resolve/src/defaultResolver.ts	88.00% <88.88%> (+2.28%)	⬆️

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[SimenB]

OK, added a condition that if there is an index file in root, we also skip the resolution. That should make the change backwards compatible.

We need to pass the node condition, but that will also require changes to the runtime to check the format of the returned file. So I'll save that for v28 (which should have full support regardless)

[github-actions]

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