import/extensions: Reverse logic of `ignorePackages` with `never`

[Drafter500]

ignorePackages flag always works in one direction, it only allows skipping extensions in packages in always mode (default), but it is impossible to make it reverse: allow having extensions for packages in never mode. Currently in combination with never, it does nothing. By the name of the flag, I expected it to stop complaining about package imports in any situation.

For example, with this config: ['error', 'never', {ignorePackages: true} ]

It should forbid extensions except for packages:

import Component from './Component'; // OK
import Component from './Component.jsx'; // Not OK
import baz from 'package/module-one'; // OK
import bizz from 'package/module-two.js'; // Should be also OK!!! 

[ljharb]

Why should that be ok?? Packages without “exports” don’t need extensions, and packages with it should be defining their paths without them as well. Can you provide an example where both forms are valid, and you want the extension on third-party code?

[Drafter500]

Why should that be ok?? Packages without “exports” don’t need extensions, and packages with it should be defining their paths without them as well. Can you provide an example where both forms are valid, and you want the extension on third-party code?

With the new version of lit some parts require extensions when importing them: https://github.com/lit/lit/issues/1986#issuecomment-870084329

The export paths are defined with extensions: https://github.com/lit/lit/blob/main/packages/lit/package.json#L18

[codecov[bot]]

Codecov Report

Merging #2231 (a65c60e) into main (fd85369) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2231   +/-   ##
=======================================
  Coverage   95.23%   95.23%           
=======================================
  Files          65       65           
  Lines        2687     2687           
  Branches      889      890    +1     
=======================================
  Hits         2559     2559           
  Misses        128      128           

Impacted Files	Coverage Δ
src/rules/extensions.js	100.00% <100.00%> (ø)

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[ljharb]

@Drafter500 in that case, the solution is for the resolve package to add "exports" support, and then for this plugin to not try to check extensions on any package with "exports" - it'd only be if a package provides two "exports" entries that point to the same file, and one had an extension and one didn't, and you also preferred the one WITH an extension, that this PR would come into play - and I'm not sure why anyone would prefer using the extension when it's possible to omit it.