RFR: 8327651: Rename DictionaryEntry members related to protection domain

[Ioi Lam]

Rename `DictionaryEntry::_pd_set` to `_package_access_cache`, and rename all related methods accordingly.

The old name was very confusing. It makes one wonder,

> Is `DictionaryEntry::_pd_set` related to the `java.lang.Class.getProtectionDomain()` of the `DictionaryEntry::_instance_klass` in any way?

(Hint, they have absolutely no relationship to each other).

I am making this PR as future changes in the Leyden project may be touching this area. I don't want the reviewers of my future PRs be confused by this code.

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This PR is mostly name changes and comment clarification.

Other than a log message change for clarification, there are no behavioral changes in this PR.

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Commit messages:
 - 8327651: Rename DictionaryEntry members related to protection domain

Changes: https://git.openjdk.org/jdk/pull/18222/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18222&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8327651
  Stats: 115 lines in 6 files changed: 8 ins; 22 del; 85 mod
  Patch: https://git.openjdk.org/jdk/pull/18222.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18222/head:pull/18222

PR: https://git.openjdk.org/jdk/pull/18222