module: fix extension lookups for top-level main

The reason is that absolute URLs do not go through extension and index
checks. By switching to an absolute path, the resolver still applies
extensions properly to the top-level main.

PR-URL: https://github.com/nodejs/node/pull/16526
Fixes: https://github.com/nodejs/node/issues/16476


Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>