[foreign-jextract] RFR: 8253841: jextract --source mode is broken

[Athijegannathan Sundararajan]

* left constant name is specially modified which is problematic.
* access flags for source constant classes should not be just ""

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Commit messages:
 - 8253841: jextract --source mode is broken

Changes: https://git.openjdk.java.net/panama-foreign/pull/362/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=362&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253841
  Stats: 9 lines in 2 files changed: 5 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/362.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/362/head:pull/362

PR: https://git.openjdk.java.net/panama-foreign/pull/362