As a user, I can specify Remote.includes with a requirements.txt

Original Pulp Redmine Issue: https://pulp.plan.io/issues/4711

This will fill out the common workflow:

1. pip install something from pypi. (pip will do dependency resolution)
2. pip freeze > requirements.txt (will include packages and all deps)
3. Create a warehouse remote, with requirements.txt used to populate the includes

This will significantly simplify the workflow, preventing the user from having to hand-parse their own includes.

Start by tracing how a warehouse remote handles Remote.includes and how requirements.txt is represented. The change is done when a requirements.txt produced by pip freeze can populate the remote's includes, simplifying the described PyPI workflow.