pip solver does not consider dependencies conditional on platform

[scottyhq]

I came across #289 which may already solve this issue, but here is another example where conda-lock (1.2.1) adds dependencies of pip packages for all platforms regardless of the lock target. For example, jupyter_server has a conditional dependency on pywintpy here for Windows ( "pywinpty;os_name=='nt'"):
https://github.com/jupyter-server/jupyter_server/blob/a55bc587e9c343509e95bd93961e27e331882834/pyproject.toml#L40

# environment.yml
channels:
  - conda-forge
dependencies:
  - jupyterlab
  - pip
  - pip:
    - jupyter-server

conda-lock lock -k env -p linux-64 still includes this Windows-only dependency, so trying to install the lockfile fails (output from the resulting lockfile below):

  - jupyterlab=3.5.1=pyhd8ed1ab_0
  - pip:
    - fastjsonschema === 2.16.2 --hash=sha256:21f918e8d9a1a4ba9c22e09574ba72267a6762d47822db9add95f6454e51cc1c
    - fqdn === 1.5.1 --hash=sha256:3a179af3761e4df6eb2e026ff9e1a3033d3587bf980a0b1b2e1e5d08d7358014
    - jsonpointer === 2.3 --hash=sha256:51801e558539b4e9cd268638c078c6c5746c9ac96bc38152d443400e4f3793e9
    - py === 1.11.0 --hash=sha256:607c53218732647dff4acdfcd50cb62615cedf612e72d1724fb1a0cc6405b378
    - pywinpty === 2.0.9 --hash=sha256:01b6400dd79212f50a2f01af1c65b781290ff39610853db99bf03962eb9a615f