Adjust phan script to run sub-projects

Adjust phan script to run sub-projects

Rather than duplicating the phan and postprocess-phan.php scripts
into extensions, this allows passing the path to the extension
(or skin, or whichever) to the phan script and it will run the analysis
for that project.

Individual projects will need a tests/phan/config.php file that defines
how the analysis will run in their project.

Bug: T153040
Change-Id: I58ad44f6a8769088e87d5d6cf1079e633c082f88