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

Should fix beeware/briefcase#2195, beeware/briefcase#669 and beeware/briefcase#381 for "macos 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
• "ios xcode": Add processing of .pth files from "app_packages". briefcase-iOS-Xcode-template#49

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 - works great in my testing. As with with VisualStudio PR, waiting until we're ready to merge all the templates before merging this one.

The pth-execution-checker was a really helpful test utility as well. That code would be a really good candidate for including in the Python-support-testbed project as a way to validate that the templates and support packages are all working together as expected.

[timrid]

That was the intention of pth-execution-checker to be included as test case in the Python-support-testbed. Feel free to fork or copy it to a repo under the beeware organization, so you dont have a dependency to a "foreign" repo, that is not under your control.