How to load latex .sty files from a subdirectory?

Question

I use some .sty-files that are not part of texlive and want to have them in a subdirectory of the folder with my main.tex. I used the following line to load the package:

\usepackage{sty/prettythesis}

This works, but compiling the main.tex with xelatex and using rubber gives me a warning:

sty/prettythesis.sty: You have requested package `sty/prettythesis',
but the package provides `prettythesis'. (page 1)

Is there a way to prevent this warning or handle this case without having to include "sty\" in all .sty-files' \ProvidesPackage command or set the TEXINPUTS environment variable (which seems not to be possible from the main.tex itself)?