Fix TS2742 false positive for self-imports in declaration files

[zaewc]

Fixes #62806

This PR fixes an issue where self-referencing imports inside .d.ts files incorrectly resolve to source files during tsc --build. This caused false TS2742 errors because inferred types appeared to reference non-portable paths in the source tree instead of the emitted declaration files.

Problem

During project reference builds, self-imports such as:

// packages/c/src/other.d.ts
import { C } from "c";

resolve "c" to src/C.ts instead of dist/C.d.ts.

This results in:
- False positive TS2742 errors
- Incorrect parentSpecifiers pointing to source directories
- Behavior that occurs only under --build, not regular tsc

Solution

A post-resolution step was added to resolveModuleName to correct self-imports in .d.ts files:
1. If the importer is a .d.ts file and the resolved module is a TS source file:
2. Walk upwards to locate the nearest package.json for the importer
3. If the module name matches the package name (indicating a self-import):
4. Recompute the expected .d.ts output path using outDir/rootDir
5. Redirect the resolved module to the correct emitted .d.ts file

This redirection only applies to .d.ts importers and does not affect normal source file resolution.

Tests

Added tests in src/testRunner/unittests/tsbuild/moduleResolution.ts verifying that:
- A self-import inside src/other.d.ts resolves to dist/index.d.ts instead of src/index.ts
- The build produces no TS2742 errors
- Trace resolution logs show the redirected path
Removing the fix causes the test to fail, confirming coverage

[zaewc]

@microsoft-github-policy-service agree

[typescript-bot]

With 6.0 out as the final release vehicle for this codebase, we're closing all PRs that don't fit the merge criteria for post-6.0 patches. If you think this was a mistake and this PR fits the post-6.0 patch criteria, please post to the 6.0 iteration issue with details (specifically, which PR and which patch criteria it satisfies).

Next steps for PRs:

• For crash bugfixes or language service improvements, PRs are currently accepted at the typescript-go repo
• Changes to type system behavior should wait until after 7.0, at which point mainline TypeScript development will resume in this repository with the Go codebase
• Library file updates (lib.d.ts etc) continue to live in this repo or the DOM Generator repo as appropriate