Context: We're trying to make Ray Data not call private Ray Core APIs. The motivation is that if Ray Data only depends on stable Ray Core interfaces, we can decouple them (e.g., stop rebuilding Ray Core in CI for Data PRs).

The env_xxx utility functions are one set of private APIs Ray Data calls in many places.

Task: Move the env_xxx to the ray._common subpackage, which defines APIs that aren't exposed to end-users but available to Ray libraries like Ray Data.