Respect existing versions when pip-installing (#608)
## Summary
When running `puffin pip-install`, we should respect versions that are
already installed in the environment. For example, if you run `puffin
pip-install flask==2.0.0` and then `puffin pip-install flask`, we should
avoid upgrading Flask. The most natural way to model this is to mark
them as "preferences".
(It's not enough to just filter those requirements out prior to
resolving, since we may not have the _dependencies_ of those packages
installed. We _could_ recursively verify this across the
`site-packages`, but that would be a larger PR.)
