[code-reflection] RFR: Move OpDeclaration to internal package

[Paul Sandoz]

Move OpDeclaration to internal package

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Commit messages:
 - Move OpDeclaration to internal package

Changes: https://git.openjdk.org/babylon/pull/475/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=475&range=00
  Stats: 778 lines in 18 files changed: 231 ins; 170 del; 377 mod
  Patch: https://git.openjdk.org/babylon/pull/475.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/475/head:pull/475

PR: https://git.openjdk.org/babylon/pull/475