custom data readers loader should not recurse further than first subdirectories of sys.path

[ikappaki]

Hi,

could you please review patch to only have the custom data reader loader only considers up to the first level subdirectories in the sys.path entries. It addresses #1135.

I've used transducers to improve performance, which required moving the custom data loaders code further down the namespace. The only function that changes is data-readers-files, which now only processes the top directory and immediate subdirectories.

I've adjusted the a test not to expect any readers to be found further down the subdirectories, andfixed a test which should have referred to test/test5`.

I've also updated the documentation to indicate the change in behavior.

Thanks

cc @mitch-kyle

[ikappaki]

Thanks @chrisrink10 for the review and suggestions! Basilisp feels noticeably quicker to bootstrap within its own project (though perhaps I'm just biased by the fix 😅).