resolveInternalSpecifier offered posix.join(dir, "index") as the
second candidate, ahead of every extension. With both dep.js and an
extensionless dep/index present, require("./dep") therefore resolved to
dep/index, a file that neither Node nor TypeScript ever selects.
Directory candidates now carry an extension. File and index ordering is
otherwise unchanged, so Node-validated cases such as dep/index.js
winning over dep.cjs keep their meaning.
Fixes #3381
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>