[foreign-jextract] RFR: 8261511: jextract fails to handle name clashes between unrelated constants

[Athijegannathan Sundararajan]

constants class is nested properly. piggybacking to fix long whitespace string in pretty printer

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Commit messages:
 - 8261511: jextract fails to handle name clashes between unrelated constants

Changes: https://git.openjdk.java.net/panama-foreign/pull/468/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=468&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261511
  Stats: 151 lines in 5 files changed: 149 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/468.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/468/head:pull/468

PR: https://git.openjdk.java.net/panama-foreign/pull/468