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]

CodSpeed Performance Report

Merging this PR will improve performance by 3.62%

_{Comparing nyurik:generated (c218d5b) with main (323dfb7)¹}

Summary

⚡ 1 improved benchmark
✅ 283 untouched benchmarks
⏩ 38 skipped benchmarks²

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	cp_large_file[16]	392.1 µs	378.4 µs	+3.62%

Footnotes

1. No successful run was found on main (548afc0) during the generation of this report, so 323dfb7 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

2. 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. ↩