gyp: put filenames in variables

This PR move source code filenames from the sources field to variables in gyp files.

With this, it becomes much easier extracting the filenames from the gyp files with tools like gypi_to_gn.py, and it significantly eases the work of compiling Node.js with another build system. Chromium was doing the exact same thing during their migration from GYP to GN.

Other than helping embedders, doing this also improves the readability of the gyp files, and actually quite a lot of places in node's gyp files are already doing it.