Maybe: Rename PossiblyDeferred type

[calvincestari]

This work is not required for delivery of the feature but is a quality of life improvement to the code with the introduction of @defer - RFC section link.

Adding support for @defer brings new meaning of the word 'deferred' to the codebase. There is an enum type named PossiblyDeferred which would cause confusion when trying to understand it’s intent. This type and its related functions should be renamed to disambiguate it from the incoming @defer related types and functions.

PossiblyDeferred is an internal type so this should have no adverse effect to users’ code.