[code-reflection] RFR: Move OpBuilder to internal package.

[Paul Sandoz]

Move OpBuilder to internal package.

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Commit messages:
 - Move OpBuilder to internal package.

Changes: https://git.openjdk.org/babylon/pull/465/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=465&range=00
  Stats: 5 lines in 3 files changed: 2 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/babylon/pull/465.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/465/head:pull/465

PR: https://git.openjdk.org/babylon/pull/465