[code-reflection] RFR: Refactor TypeDefinition to ExternalizedTypeElement

[Maurizio Cimadamore]

On Thu, 2 May 2024 18:51:59 GMT, Paul Sandoz <psandoz at openjdk.org> wrote:

> Refactor `TypeDefinition` to `ExternalizedTypeElement` and enclose within `TypeElement`.
> 
> We may, in a subsequent PR, add a `ExternalizableTypeElement` that encloses `ExternalizedTypeElement`, thereby aligning with externalizable operations.

Looks good

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Marked as reviewed by mcimadamore (Reviewer).

PR Review: https://git.openjdk.org/babylon/pull/72#pullrequestreview-2037781734