Write analysis for practice exercise `split-second-stopwatch`

[jiegillet]

split-second-stopwatch is an exercise supposed to focus on practicing opaque types.

• [essential] make sure that Stopwatch is not defined with type alias
• [essential] make sure that Stopwatch is not exported with (..)

Keep in mind that other types could potentially be defined with a type alias, they must not trigger any warning.
This will require custom rules.