Alright, checked some more, and the explicit relative imports seem to indeed work correctly. __package__ is also very useful.
What I'm now wondering about is the first line in my example, since it doesn't explicitly import relatively, but just from the regular PATH locations. I'm almost 100% certain this was different in pysta 1.4, because this script's PATH-relative imports worked just fine, even if the script and its modules were in a subfolder. Now (at least I think it's caused by the 1.5 update) this no longer works, and the modules either need to be in a regular PATH location, or the folder name needs to also be given in the import statements.
Figured it out... apparently SOMEHOW my os.environ["PATH"] got messed up and no longer had "." in it. Added that back in, restarted the app, and everything worked perfectly. Sorry for the trouble.
Though I'd be very interested in what did change my PATH in the first place.