fix: resolve aliases pointing at a package directory ending in .js - #21542

…ts own extension (#641)

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**Summary**

`ExtensionAliasPlugin` marks every candidate `fullySpecified: true` and
then refuses to fall back, so a request ending in an aliased extension
can only ever resolve as an exact file — never as a directory or via
`package.json#main`. With the source extension listed among its own
aliases (`{ ".js": [".js", ".ts"] }` — the documented TypeScript recipe,
and what `experiments.typescript` sets in webpack) that entry is meant
to keep the original request valid, but it is tried fully specified too,
so any package or directory whose name ends in `.js` becomes
unreachable. This makes the identity entry mean what it says: after the
fully specified candidates fail, fall through to normal resolution. A
mapping that drops the source extension (`{ ".js": [".ts"] }`, `{
".mjs": ".mts" }`) stays strict.

Refs webpack/webpack#21541. That regression is fixed on the webpack side
in webpack/webpack#21542 and does not need this; this closes the same
failure for projects that genuinely enable TypeScript today, and ahead
of TypeScript becoming a default.

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**What kind of change does this PR introduce?**

fix

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**Did you add tests for your changes?**

Yes — three cases in `test/extension-alias.test.js` (directory named
`dir3.js`, an aliased absolute path to a `pkg.js` package, and a strict
mapping that must keep failing) plus the matching fixtures.

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**Does this PR introduce a breaking change?**

No — it only turns previously failing resolutions into successes;
exact-file candidates are still tried first, so nothing that resolves
today resolves differently.

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**If relevant, what needs to be documented once your changes are merged
or what have you already documented?**

`extensionAlias` docs should say that listing an extension among its own
aliases keeps the original request resolvable in its normal form, while
omitting it makes the mapping strict.

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**Use of AI**

AI (Claude Code) was used to trace the failure, draft the fix and tests,
and verify them; the diagnosis and the final diff were reviewed by a
human before submitting.

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