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GH-40431: [C++] Move key_hash/key_map/light_array related files to internal for prevent using by users (#40484)

### Rationale for this change

These files expose implementation details and APIs that are not meant for third-party use. This PR explicitly marks them internal, which also avoids having them installed.

### Are these changes tested?

By existing builds and tests.

### Are there any user-facing changes?

No, except hiding some header files that were not supposed to be included externally.

* GitHub Issue: #40431

Lead-authored-by: ZhangHuiGui <hugo.zhang@openpie.com>
Co-authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>