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.  It also sets us up to
have a 'plugins' directory where xctool can dynamically load bundles
that implement new actions or reporters.