Add processing of .pth files from "app_packages".

[timrid]

As described here, the app_packages should be added via site.addsitedir() so that .pth files are executed correctly.

I tested it with a minimal example project I made for this: pth-execution-checker

In contrast to my previous attempts with the .pth files on iOS (see here), now the output of a print() inside the .pth files also works, since site.addsitedir() is called after the initialization of the NSLog and not before.

Should fix beeware/briefcase#2195, beeware/briefcase#669 and beeware/briefcase#381 for "ios xcode".

There are also a similar PRs for:

• "windows visualstudio": Add processing of .pth files from "app_packages". briefcase-windows-VisualStudio-template#52
• "linux flatpak": Add processing of .pth files from "app_packages". briefcase-linux-flatpak-template#58
• "linux system": Add processing of .pth files from "app_packages". briefcase-linux-system-template#36

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[freakboy3742]

Awesome - as with with VisualStudio PR, waiting until we're ready to merge all the templates before merging this one.