What you see in the browser is front end , and the PHP is backend. You will not be able to download any PHP files using a browser tool.
My suggestion was really for tracking down where the calls to googleapis are coming from from the browser. You can often tell from the surrounding source. When I looked at the code here on Moodle.org, its coming from the theme.
Really the best way though is what you were doing. Search the source tree for your Moodle site. Most IDEs have that feature. I am sure its possible with command line tools and other things too. When I did that using PHP Storm I got lots of hits from the lib/google directory. And from the Google Docs repository. That means you would need to find out which services are using the Google APIs and turn them off if possible.