[code-reflection] Integrated: Initial prep for swapping callgraphs for Op.Module

[Gary Frost]

These are initial steps towards swapping existing callgraph implementation with CoreOp.ModuleOp

Basically adding ModuleOpWrapper

I also took the opportunity of cleanup up some code for converting MethodRef -> Method.

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Commit messages:
 - Initial prep for swapping callgraphs for Op.Module

Changes: https://git.openjdk.org/babylon/pull/236/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=236&range=00
  Stats: 202 lines in 9 files changed: 154 ins; 7 del; 41 mod
  Patch: https://git.openjdk.org/babylon/pull/236.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/236/head:pull/236

PR: https://git.openjdk.org/babylon/pull/236