Dynamic setting of extractPackages

[timrid]

This PR adds the ability to dynamically set packages to be extracted via extractPackages via a extract-packages.txt file. This adds the possibility in briefcase to define these packages in briefcase build and not in briefcase create.

To set breakpoints when debugging via VSCode (see beeware/briefcase#2351), the packages to be debugged must first be extracted so that debugpy can map the breakpoints set on the host PC to those on the Android device via the path mappings. For this mapping, however, the .py files must be located on the Android file system and therefore be extracted. However, since this extraction leads to a longer startup time, this should only be done for debug builds via briefcase build --debug. Currently it is only possible to specify extractPackages during briefcase create. But briefcase create has no --debug option, and I dont think it is worth adding it to create.

briefcase-repo: https://github.com/timrid/briefcase
briefcase-ref: feature/debugger-support-android

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

[mhsmith]

This PR looks good to me, but because of the target_version bump, it can't be merged until beeware/briefcase#2351 is ready.