[code-reflection] RFR: Explicit op factory implementations

[Paul Sandoz]

Instead of using reflection to construct operation factories for `JavaOp` and `CoreOp` use explicit methods. These methods were generated with some helper code that is commented out.

This enables constructors/methods that construct from an externalized op to become package private.

In a subsequent PR we can remove the class value factory implementation and update the examples to use explicit factories or copy the class value implementation. Keeping the annotation `OpDeclaration` seems useful for now, but may be better if enclosed within `Op`, and it may be made internal later on.

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Commit messages:
 - Explicit op factory impls

Changes: https://git.openjdk.org/babylon/pull/467/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=467&range=00
  Stats: 328 lines in 3 files changed: 174 ins; 0 del; 154 mod
  Patch: https://git.openjdk.org/babylon/pull/467.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/467/head:pull/467

PR: https://git.openjdk.org/babylon/pull/467