Explore how we can leverage pip install's new --dry-run flag

[jeffwidman]

pip just added a pip install --dry-run flag.

We should explore how we can leverage this, especially because:

1. Python is the slowest native helper that we have
2. We only care about the resolver output, not the installed artifacts.

Tasks

• Research how the flag works... under the covers, does it skip any steps compared to a full install?
• Research whether we can ingest the output in a machine-parsable format
• Can we leverage this in how we call pip to save time?

[jeffwidman]

See also:

• Explore decoupling from pip's internals, using the new --dry-run and --report flags jazzband/pip-tools#1654
• Switch to leveraging pipenv's new pipenv upgrade command #6836

[jeffwidman]

Related, we may also be able to leverage the pip check command to validate the existing list of requirements... may or may not be useful, not quite sure yet.

See the 3rd example here: https://pip.pypa.io/en/stable/cli/pip_check/