RFR: JDK-8308239: Tighten up accessibility of nested classes in java.lang.invoke

[Joe Darcy]

Tightening up accessibility of a few nested classes. There is no practical impact on source compatibility -- source uses within the package still work, but by default the ability to reflectively call methods on these classes is disabled.

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Commit messages:
 - JDK-8308239: Tighten up accessibility of nested classes in java.lang.invoke

Changes: https://git.openjdk.org/jdk/pull/14021/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14021&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308239
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/14021.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14021/head:pull/14021

PR: https://git.openjdk.org/jdk/pull/14021