module: fix loading from global folders on Windows

Code was calculating $PREFIX/lib/node relative to process.execPath, but
on Windows process.execPath is $PREFIX\node.exe whereas everywhere else
process.execPath is $PREFIX/bin/node (where $PREFIX is the root of the
installed Node.js).

PR-URL: https://github.com/nodejs/node/pull/9283


Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: João Reis <reis@janeasystems.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>