A selected package exports target names one file, and Node tries no
extension or directory index for it. REA still resolved it like a relative
specifier, so `exports: "./file"` with only `file.js` present, or
`"./folder"` with only `folder/index.js`, reported a resolved import that
Node rejects under both import and require.
Resolve an exports target only to its exact file and otherwise report
not-found with a limitation naming the target. Legacy main entries keep
their extension and index fallbacks.
Fixes#1174
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>