refactor: move generated code into sub-mod

[nyurik]

generated/included code should always be inside its own mod to avoid accidentally adding new names to the current scope. This also allows clean way of adding new attributes to the whole file - something not possible to do to a single include! statement.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/misc/usage_vs_getopt (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/shuf/shuf-reservoir (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/sort/sort-stale-thread-mem (passes in this run but fails in the 'main' branch)

[codspeed-hq]

Merging this PR will degrade performance by 6.7%

❌ 2 regressed benchmarks
✅ 286 untouched benchmarks
⏩ 38 skipped benchmarks¹

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	unexpand_large_file[10]	397.7 ms	426.3 ms	-6.7%
❌	Simulation	unexpand_many_lines[100000]	189.7 ms	203.4 ms	-6.7%

---

_{Comparing nyurik:generated (211faa3) with main (ed05c6e)²}

Footnotes

1. 38 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

2. No successful run was found on main (aadb7d6) during the generation of this report, so ed05c6e was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/follow-name (passes in this run but fails in the 'main' branch)

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/pr/bounded-memory. tests/pr/bounded-memory is passing on 'main'. Maybe you have to rebase?
Congrats! The gnu test tests/factor/t29 is no longer failing!
Congrats! The gnu test tests/factor/t33 is no longer failing!
Note: The gnu test tests/seq/seq-epipe is now being skipped but was previously passing.