sg: redesign checks functionality

Created by: mrnugget

This does multiple things to fix https://github.com/sourcegraph/sourcegraph/issues/31043

• Move Sourcegraph-independent check functionality into internal/check (including helpers like check.Version, check.Any, etc.)
• Remove the previous run.Check and removes the distinction between built-in and config checks
• Remove all checks from sg.config.yaml. Old sg versions will run into "WARNING: check not found" messages, so it's kinda backwards-compatible
• Moves all Sourcegraph and sg-related check code into a single file, checks.go
• Adds a helper to safely get a check by name
• Fixes #31039 (closed) by checking that Docker is running, but only in sg doctor, not in setup.
• Changes output of sg doctor to be more readable
• Move some functionality into usershell so the code is in a leaf package

Test plan

• Run go run . doctor locally with Redis/Postgres/Docker on and off
• Run go run . setup locally with Redis/Postgres/Docker on and off