Port tryLoadInputFileForPath from TypeScript codebase

[AlCalzone]

Fixes: #1345
and improves several baselines of existing tests
I don't fully grasp some of the error diffs, but the other changes at least look like an improvement to me.

Disclaimer: I burned a couple of tokens to get Claude to port this, since I don't understand enough of the codebase to make this change on my own - so please check that no nonsense slipped in.

[Copilot]

Pull Request Overview

This PR ports the TypeScript tryLoadInputFileForPath logic into our Go resolver, adds two helpers for mapping output files back to source inputs, and updates a slew of compiler tests and baselines to reflect the new “self-name + outDir” behavior.

• Implement tryLoadInputFileForPath support for package self-names with outDir/declarationDir, guessing common source directories and mapping outputs back to inputs
• Add helpers getPossibleOriginalInputExtensionForExtension and getOutputDirectoriesForBaseDirectory
• Introduce a new test (subpathImportJS.ts) and update many baseline files under testdata/baselines to match the new resolution behavior

Reviewed Changes

Copilot reviewed 61 out of 61 changed files in this pull request and generated 2 comments.

File	Description
internal/module/resolver.go	Ported tryLoadInputFileForPath logic from TS; added helpers for extension and directory guesses
testdata/tests/cases/compiler/subpathImportJS.ts	New test exercising package import maps and self-paths
testdata/baselines/reference/...	Updated expected outputs/errors/symbols/types to match new resolution results

[JoostK]

This is pending in #1302

[jakebailey]

Yeah, I don't think we need this PR, given #1302 is the same? Just, a discussion about if we are keeping this logic or if there's some better thing we can do?

[jakebailey]

Going to close this for clarity, but if you see something in my PR that your PR did better, absolutely tell me.

[AlCalzone]

I trust that you have a clear idea what you want to achieve - I merely told Copilot to port the TypeScript implementation as close as possible.

[jakebailey]

I have to admit, I also did that (which succeeded).