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 using a manually created .pth file that executed a .py file with a print. The output of the print was visible in the console with briefcase run windows app.

I also added pywin32 as a dependency in my application and tested it with the following code:

import win32ui
win32ui.MessageBox("I am working now", "Hello pywin32")

Should fix beeware/briefcase#2195, beeware/briefcase#835, beeware/briefcase#669 and beeware/briefcase#381 for "windows app".

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]

Thanks for the contribution. Unfortunately, while this does work, it's not the best approach for a fix, so I'm going to close this PR. See beeware/briefcase-windows-VisualStudio-template#52 for more discussion of the reasons why.