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%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

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

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

Performance Changes

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

---

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

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. ↩

[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.