module: support main w/o extension, pjson cache

This adds support for ensuring that the top-level main into Node is
supported loading when it has no extension for backwards-compat with
NodeJS bin workflows.

In addition package.json caching is implemented in the module lookup
process.

PR-URL: https://github.com/nodejs/node/pull/18728


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>