Imports conditions

[emmatown]

This implements the design from #542 (comment) under an experimental flag importsConditions. There's still some stuff I probably want to change here (hence why it's under an experimental flag) but I'd like to try it out and see how it is to use first.

A thing I didn't mention in the PR comment linked above that I've done here is for imports fields like this:

{
  "imports": {
    "#a": {
      "worker": "./src/a.js",
      "browser": "./src/b.js",
      "default": "./src/a.js"
    }
  }
}

We'll only generate two bundles for each format rather than three since the worker and default conditions resolve to the same place. (we use the shortest combination of conditions to name bundles)

Also, in the generated .d.ts files, we re-write imports from #something to the resolved thing with the default condition applied. This is so that the .d.ts files work in moduleResolution: node (though of course, when authoring packages, you need to use bundler or nodenext so that #something can be resolved).

In the future, I'm thinking of having essentially two .d.ts outputs, one with #something replaced and one where it's not replaced, this would allow having different types for different conditions. I don't think that by itself would be that useful tbh since I'm guessing that most package consumers will not be configuring customConditions in TS. Though what would be quite useful is the types@semver-range-of-ts-version magical condition thing, specifically, we could essentially polyfill it for moduleResolution: node via typesVersions without users having to deal with typesVersions. One slight problem with this is that without isolatedDeclarations, you could accidentally rely on type information from a particular implementation of #something and therefore generate the wrong .d.ts files so I'm thinking if/when we implement this part, it'll probably stay experimental until isolatedDeclarations exists. (we could of course also just generate .d.ts files for each set of conditions but that would be unacceptably slow imo)

Also, one random thing that's interesting with this design (though I don't think I'd say it's really a problem) is that you can conditionally have different exports for different conditions except that you couldn't conditionally have a default export. (the implementations of a default export could be different for different conditions but you couldn't have a default export under some conditions and not under others)

cc @Andarist

Closes #542

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Latest commit: 497bf6a

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[codecov]

Codecov Report

Patch coverage: 93.95% and project coverage change: +0.01 🎉

Comparison is base (41a8a4f) 91.79% compared to head (56c9856) 91.80%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #551      +/-   ##
==========================================
+ Coverage   91.79%   91.80%   +0.01%     
==========================================
  Files          38       41       +3     
  Lines        1730     2002     +272     
  Branches      493      585      +92     
==========================================
+ Hits         1588     1838     +250     
- Misses        133      154      +21     
- Partials        9       10       +1     

Impacted Files	Coverage Δ
packages/cli/src/build/utils.ts	100.00% <ø> (ø)
packages/cli/src/project.ts	85.13% <ø> (ø)
packages/cli/src/validate.ts	88.23% <ø> (ø)
packages/cli/test-utils/index.ts	96.31% <81.81%> (-1.09%)	⬇️
packages/cli/src/build/config.ts	78.94% <83.33%> (-2.22%)	⬇️
...eclarations/get-used-declaration-with-replacing.ts	85.18% <85.18%> (ø)
packages/cli/src/package.ts	95.42% <92.85%> (-0.38%)	⬇️
packages/cli/src/imports.ts	95.19% <95.19%> (ø)
packages/cli/src/utils.ts	98.62% <95.74%> (-1.38%)	⬇️
packages/cli/src/dev.ts	95.53% <96.07%> (-0.26%)	⬇️
... and 10 more

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