[foreign-jextract] RFR: 8253465: jextract should generate constant classes as package private

[Athijegannathan Sundararajan]

generated constant classes are package private now.

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Commit messages:
 - 8253465: jextract should generate constant classes as package private

Changes: https://git.openjdk.java.net/panama-foreign/pull/352/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=352&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253465
  Stats: 9579 lines in 10 files changed: 2348 ins; 1990 del; 5241 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/352.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/352/head:pull/352

PR: https://git.openjdk.java.net/panama-foreign/pull/352