RFR: 7903321: Use a set instead of a map to record methods in class 'MethodGroup'

[Guoxiong Li]

Hi all,

This patch changes the type of the field `MethodGroup#methods` from a map to a set so that it is more suitable for its meaning.

Thanks for the taking the time to review.

Best Regards,
-- Guoxiong

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Commit messages:
 - CODETOOLS-7903321

Changes: https://git.openjdk.org/jmh/pull/79/files
 Webrev: https://webrevs.openjdk.org/?repo=jmh&pr=79&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903321
  Stats: 14 lines in 1 file changed: 2 ins; 3 del; 9 mod
  Patch: https://git.openjdk.org/jmh/pull/79.diff
  Fetch: git fetch https://git.openjdk.org/jmh pull/79/head:pull/79

PR: https://git.openjdk.org/jmh/pull/79