Better messages for platform selectors in environment.yml

[AlbertDeFusco]

Since cona-lock supports preprocessing selectors, they also work for conda-project

channels:
  - conda-forge
dependencies:
  - python=3.9
  - pywin32 # [win]

When running conda project lock the lock file is correctly generated, but the messages could be a little clearer or perhaps the warnings are not all that useful.

❯ conda project lock
Locking dependencies for environment default on platforms linux-64, win-64, osx-64, osx-arm64: ◜ WARNING:conda_lock.src_parser.selectors:filtered out line `  - pywin32 # [win]` due to unmatchable selector
WARNING:conda_lock.src_parser.selectors:filtered out line `  - pywin32 # [win]` due to unmatchable selector
WARNING:conda_lock.src_parser.selectors:filtered out line `  - pywin32 # [win]` due to unmatchable selector
Locking dependencies for environment default on platforms linux-64, win-64, osx-64, osx-arm64: done