I think catkin_simple should find_package <buildtool_depends>, not just <build_depends>? Or do I misunderstand how buildtool_depends is meant to be used.

In my case I have a package that adds some cmake macros (similar to catkin_simple itself) which I use in the CMakeLists.txt of a package that buildtool-depends on that cmake-macro-package. My current workaround is to specify it as build-depend instead.