Don't expose isPromise as an internal module

Editor assessment

The subject states the encapsulation change: isPromise no longer exposed as an internal module. The refactor type fits the visibility reduction, and the absent scope is tolerable. No body is present, which is acceptable because the subject names the module and the removal of exposure completely, though the internal consumers consolidated would have been worth a line. The PR reference is well-formed. Imperative mood, no trailing period, compact length. A precise, self-sufficient refactor message that needs no diff to understand.