import-is-undefined warning issued on TS import equals declaration

[jakebailey]

v0.18.15 moved this error to be enabled as warning by default; I noticed when compiling TypeScript that we hit this warning, but it doesn't look to be correct:

▲ [WARNING] Import "ArrayOrSingle" will always be undefined because there is no matching export in "src/harness/_namespaces/FourSlashInterface.ts" [import-is-undefined]

    src/harness/fourslashImpl.ts:9:42:
      9 │ import ArrayOrSingle = FourSlashInterface.ArrayOrSingle;
        ╵                                           ~~~~~~~~~~~~~

TS will elide this statement as it does other type-only declarations: Playground Link

I'm not sure how analyzable this is within esbuild; probably, this should just be a "normal" import on our end anyway, it's just leftover from the module conversion.