RFR: 7903925: Only the main generated header class should be public

[Nizar Benalla]

Please review this patch to make the different header classes jextract generates (e.g. foo_h, foo_h_1, foo_h_2, ...) into package-private classes, leaving the main header class (without prefixes) as the only public class.

While reading the sources, I noticed what seems like small bug in `SourceFileBuilder` introduced in https://github.com/openjdk/jextract/commit/a699148238fad919390d67eeb73805e780a518e8 so I fixed it as part of this patch.

TIA.

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Commit messages:
 - consider numbered header classes as part of the implementation

Changes: https://git.openjdk.org/jextract/pull/273/files
  Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=273&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903925
  Stats: 23 lines in 2 files changed: 8 ins; 0 del; 15 mod
  Patch: https://git.openjdk.org/jextract/pull/273.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/273/head:pull/273

PR: https://git.openjdk.org/jextract/pull/273