Adjust cl imports

[ThomasHepworth]

As discussed, here's a slightly cleaner layout for our comparisons so that we obscure some of the imports/exports to the users.

Some background:
At present, when you use:

import duckdb.duckdb_comparison_level_library as cll

all of our base classes are imported, alongside the classes we wish end users to take advantage of.

This means it’s possible for people to use:

cll.ElseLevelBase

by accident, should they not read the docs or know what they’re doing.

This used to be more of an issue than it is now, where the _level functions were imported and used in duckdb_comparison_library.py and would cause issues if you weren’t paying attention.

[github-actions]

Test: test_2_rounds_1k_duckdb

Percentage change: -13.4%

	date	time	stats_mean	stats_min	commit_info_branch	commit_info_id	machine_info_cpu_brand_raw	machine_info_cpu_hz_actual_friendly	commit_hash
849	2022-07-12	18:40:05	1.89098	1.87463	splink3	c334bb9	Intel(R) Xeon(R) Platinum 8370C CPU @ 2.80GHz	2.7934 GHz	c334bb9
1659	2023-05-25	17:13:40	1.64783	1.62271	(detached head)	84bf606	Intel(R) Xeon(R) CPU E5-2673 v4 @ 2.30GHz	2.2947 GHz	84bf606

Test: test_2_rounds_1k_sqlite

Percentage change: -6.0%

	date	time	stats_mean	stats_min	commit_info_branch	commit_info_id	machine_info_cpu_brand_raw	machine_info_cpu_hz_actual_friendly	commit_hash
851	2022-07-12	18:40:05	4.32179	4.25898	splink3	c334bb9	Intel(R) Xeon(R) Platinum 8370C CPU @ 2.80GHz	2.7934 GHz	c334bb9
1661	2023-05-25	17:13:40	4.07406	4.00453	(detached head)	84bf606	Intel(R) Xeon(R) CPU E5-2673 v4 @ 2.30GHz	2.2947 GHz	84bf606

Click here for vega lite time series charts

[ThomasHepworth]

Another potential layout - if you'd prefer that _comparison_imports.py was cleaned up:

.
{linker}_linker.py
├── {linker}_helpers
│   └── comparison_level_library.py
│   └── comparison_template_library.py
│   └── comparison_library.py

and then adjust the import paths accordingly.

[RossKen]

I think I prefer the current structure rather than your alternative ☝️

[RossKen]

I really like this change, and all seems to work as expected.

Just one thing to change - can you update the comparison and comparison level dev guides to reflect the changes of the folder structure? There are only a couple of references, then I am happy to approve.

[RossKen]

Sorry didn't mean to approve - there are tests now failing