pw_env_setup: Create more options

Split up environment config files so it's easier to set up just a subset
of CIPD packages. Splitting out Bazel and Python for now.

Change-Id: I39a5a01e32f3f8cbddea401ecd5887576040da81
Bug: 392
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/47088
Pigweed-Auto-Submit: Rob Mohr <mohrr@google.com>
Reviewed-by: Keir Mierle <keir@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>