More robust xcodeproj finding

Make the prepare command use the Podfile to find the `xcodeproj` instead of naively expecting it to be next to the Podfile.

Some projects don't keep their `xcodeproj` next to their Podfile, and if they don't, they'll specify a path for it in their Podfile. Use that.

Also, use `PODS_ROOT` instead of `SRCROOT` to link to the python script. The `Pods` directory is no guaranteed to be in the `SRCROOT`