[code-reflection] RFR: Refactor type element

[Paul Sandoz]

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.

-------------

Commit messages:
 - Refactor type element.

Changes: https://git.openjdk.org/babylon/pull/70/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=70&range=00
  Stats: 1069 lines in 67 files changed: 167 ins; 177 del; 725 mod
  Patch: https://git.openjdk.org/babylon/pull/70.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/70/head:pull/70

PR: https://git.openjdk.org/babylon/pull/70