[code-reflection] RFR: Reactor TypeDefinition to ExternalizedTypeElement

[Paul Sandoz]

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.

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Commit messages:
 - Reactor TypeDefinition to ExternalizedTypeElement

Changes: https://git.openjdk.org/babylon/pull/72/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=72&range=00
  Stats: 325 lines in 24 files changed: 112 ins; 117 del; 96 mod
  Patch: https://git.openjdk.org/babylon/pull/72.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/72/head:pull/72

PR: https://git.openjdk.org/babylon/pull/72