Add requirements.txt to each project

Make life easier for those who start from fresh virtual environments.

In the meantime, `git grep import` should not be too overwhelming.

Start by listing the project directories and using the issue's suggested `git grep import` command to identify dependencies. Determine which directories count as projects, then ensure each has a requirements.txt covering its imports; done means fresh virtual environments can install the needed dependencies for every project.

Written by the indexing model from the issue text.