module: fixes #16476 ensuring extension lookups for top main

The reason as stated in https://github.com/nodejs/node/issues/16476#issuecomment-339637581 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.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

esmodules