[jdk21u-dev] RFR: 8310913: Move ReferencedKeyMap to jdk.internal so it may be shared

[Aleksey Shipilev]

Unclean backport to match the codebases between mainline and JDK 21, simplifying maintenance. It also implicitly fixes [JDK-8323505](https://bugs.openjdk.org/browse/JDK-8323505), since this move includes a few touchups to the test.

The uncleanliness comes from `MethodType.java` conflicts that does not have [JDK-8310849](https://bugs.openjdk.org/browse/JDK-8310849) in JDK 21. So it yields a few contextual conflicts. Since affected hunks are just removals, I removed them by hand.

There is a follow-up, [JDK-8325255](https://bugs.openjdk.org/browse/JDK-8325255), but it does not seem to affect anything yet, since there are no `ReferenceKeySet.add` calls anywhere in sight. We can backport that separately.

Additional testing:
 - [x] MacOS AArch64 Server fastdebug, `jdk/internal/util` tests pass
 - [x] MacOS AArch64 Server fastdebug, `java/lang/invoke` tests pass
 - [x] Linux AArch64 Server fastdebug, `all` tests pass

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Commit messages:
 - Backport 6af0af593446bc33dc94bbf7334c325c4ac0ac0f

Changes: https://git.openjdk.org/jdk21u-dev/pull/342/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=342&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310913
  Stats: 1522 lines in 13 files changed: 887 ins; 627 del; 8 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/342.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/342/head:pull/342

PR: https://git.openjdk.org/jdk21u-dev/pull/342