Move to new build output format.

[freakboy3742]

Move to the new build/dist output format.

Refs beeware/briefcase#1107.

Briefcase-Repo: https://github.com/freakboy3742/briefcase.git Briefcase-Ref: build-dist

PR Checklist:

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