Make cli.api the only public module

[sfc-gh-turbaszek]

Pre-review checklist

• [x] I've confirmed that instructions included in README.md are still correct after my changes in the codebase.
• [x] I've added or updated automated unit tests to verify correctness of my new code.
• [x] I've added or updated integration tests to verify correctness of my new code.
• [x] I've confirmed that my changes are working by executing CLI's commands manually.
• [x] I've confirmed that my changes are up-to-date with the target branch.
• [ ] I've described my changes in the release notes.
• [x] I've described my changes in the section below.

Changes description

Make snowflake.cli.api the only public API to make sure external developers won't use any internal implementation.