[api-extractor] Report unresolvable inline import paths in .d.ts rollups

[Phạm Mạnh Lực (MLuc24)]

Summary

Fixes #4507

Per Pete Gonzalez (@octogonz)'s conclusion on that issue — "Let's improve the error message: API Extractor could analyze the .d.ts output and identify import paths such as ../Bar that clearly don't make sense in a rollup, and report a more intuitive error message" — this adds a dedicated ae-unresolved-import-path message for that case.

Today an unresolvable inline import() type is emitted into the rollup verbatim. The generated .d.ts then contains a relative path that points nowhere, so it does not compile for consumers, and the only hint is an ae-forgotten-export warning that names the symbol rather than the real problem.

Details

In DtsEmitHelpers.modifyImportTypeSpan, everything happens under if (referencedEntity). When the entity lookup fails there is no else, so the span keeps its original text and the path survives into the rollup. The new else if inspects the import's module specifier and, when it is relative, reports the message against the same declaration.

The check is deliberately narrow: only a specifier starting with . is reported. A bare specifier that fails to resolve is a different situation (the rollup emits a real import for it), and the resolving case is untouched — see the negative test below.

Two choices I would be happy to change: the message id name ae-unresolved-import-path, and leaving it at the default warning severity rather than adding an entry to api-extractor-defaults.json. Say the word and I will adjust either.

I did not add a case under build-tests/api-extractor-scenarios, because I could not regenerate the committed expected outputs locally (see below). Happy to add one if you would like it in this PR.

How it was tested

Built the contrived example from the issue against @microsoft/api-extractor 7.58.12 — Bar.ts exporting an enum, foo/Foo.ts declaring export type Foo = import('../Bar').Bar.A;, and index.ts re-exporting Foo.

Before, the rollup silently ends up unusable:

$ npx api-extractor run --local
Warning: lib/foo/Foo.d.ts:1:1 - (ae-forgotten-export) The symbol "Bar" needs to be exported by the entry point index.d.ts
API Extractor completed successfully

$ cat out-rollup/rollup.d.ts
export declare type Foo = import('../Bar').Bar.A;

With this change applied, the cause is reported:

Warning: lib/foo/Foo.d.ts:1:1 - (ae-unresolved-import-path) The inline import path "../Bar" could not be resolved,
  so it would be emitted unchanged into the .d.ts rollup, where it does not resolve to anything.
  Import the symbol at the top of the file instead of using an inline import() type.

Negative test, to confirm it does not fire on inline imports that resolve normally — changing Foo.ts to export type Foo = import('../Bar').Bar;:

$ npx api-extractor run --local
Warning: ... (ae-forgotten-export) ...
API Extractor completed successfully          # no ae-unresolved-import-path

$ cat out-rollup/rollup.d.ts
declare enum Bar {
    A = "A"
}
export declare type Foo = Bar;

Both runs were performed by applying this change to the published package's compiled output, since I could not run the monorepo's own rush build locally. The API report in common/reviews/api/api-extractor.api.md was updated by hand for the same reason, so it is worth confirming in CI that it matches.

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