[code-reflection] RFR: Refactor type element

[Maurizio Cimadamore]

On Wed, 1 May 2024 21:42:41 GMT, Paul Sandoz <psandoz at openjdk.org> wrote:

> Rename `TypeElement` to `CodeType`, thereby avoiding confusion with `javax.lang.model.element.TypeElement` (`CodeType` is closer to `javax.lang.model.type.TypeMirror` and `java.lang.reflect.Type`).
> 
> Rename `TypeDefintion` to `ExternalizedCodeType` and enclose within `CodeType`.
> 
> It's possible to go further and define a subtype `ExternalizableCodeType` that supports externalization to `ExternalizedCodeType`, copying the pattern for operations. This requires some adjustments to the externalization of types that may be composed of other posssibly non-externalizable types e.g. `FunctionType`. We can consider this separately to this PR.

Marked as reviewed by mcimadamore (Reviewer).

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PR Review: https://git.openjdk.org/babylon/pull/70#pullrequestreview-2035397982