Fix resource/UI case confusion breaking non-VPK usage on case-sensitive OS

Using hud_src in tf/custom/ directly fails on a case-sensitive host (e.g. Linux and some configurations of OS X), due to most files being in resource/UI. It looks like some parts of valve's pseudo-case-insensitivity hacks don't apply here: https://github.com/TheKins/frankenhud/tree/master/hud_src/resource

Moving everything into ui (lowercase) fixes the issue