remove some ._filenames uses in favor of .filenames

[sappelhoff]

following #12843

I think it'd be good to reduce the use of private attributes in our own code base as well, if we might as well use the public attribute.

[larsoner]

I don't have time to look deeply yet, just a quick comment that in the PR where we made the switch to paths and a public attribute, at some point I used git grep and tried to change the ones that I thought made sense to change, and left ones behind that didn't make sense to change or couldn't be changed. It's entirely possible I missed a few or I was wrong about what couldn't be changed though!

[sappelhoff]

@larsoner @cbrnr the fix in c40c6f4 seems to have solved the failing tests. I believe the changes here can truly be converted to use the public attribute.

[cbrnr]

That's great and 👍 for merging, but I am still a bit concerned why only raw._filenames and not raw.filenames works, as these two are supposed to point to the same object, or no?