npm_execpath can point through a symlink, and the manifest sits beside
the resolved CLI, never beside the link: resolve the realpath before
looking for package.json. Layouts without a readable manifest now fall
back to the pre-fix child probe instead of aborting with ENOENT.
Move the regression test to tests-js/node-deps.test.mjs (the module's
own suite) and cover the symlinked-execpath and fallback lanes there.
646c3c8ad5