xctool's build products should have a bin, lib, libexec dir structure.

This will give us something that looks like --

../xctool/bin/xctool
../xctool/lib/otest-shim-ios.dylib
../xctool/lib/otest-shim-osx.dylib
../xctool/lib/xcodebuild-fastsettings-shim.dylib
../xctool/lib/xcodebuild-shim.dylib
../xctool/libexec/mobile-installation-helper.app
../xctool/libexec/otest-query-ios
../xctool/libexec/otest-query-osx

This will make xctool play nicer with Homebrew.