[split] Uc-not-first suggestiion

@mtvee has a point. Generally speaking, breaking BC is not good. And I don't see enough reason to drop non-capitalized files.

Having 2 files with the same name is not enough reason. Because it is okay that we have a priority rule.
For example, if there is `Welcome.php`, CI chooses it. If there is not, CI searches for `welcome.php`.

But personally, I like that the file name and case matches that of the class it holds.
So I'm satisfied with the current implementation.

When you have a lot of files, it is not hard to change the filenames. I wrote a tool.
https://github.com/kenjis/codeigniter3-filename-checker
But if I had 1000 file running site, and it does not have enough test code, I would not be happy.

Thank you