Finish dropping --skip-lock so we can move forward with migrating away from Requirementslib

[matteius]

Issue description

A simplified architecture for pipenv would be:

--skip-lock is problematic because it requires also being able to generate the install lines from the Pipfile. Additionally given the other bugs around this flag, the recommendation is if you want to skip the lock and install something for testing just run pipenv run pip install If you want to do this from the Pipfile run pipenv requirements to generate a requirements file and then run pipenv run pip install -r requirements.txt

This will also improve the security of pipenv because some users may be using the --skip-lock flag in their CI builds.