Trust xcrun now that we error out early for bad xcode-select paths

Since ae177adb2bd55ee5ad6367e7639c4cf0c774b63a, we can safely assume
that xcrun works, and a functioning xcrun will search dev_tools_path and
xctoolchain_path, so we can stop doing extra work here.

On CLT-only 10.7 and 10.8, xcrun will not work, but all the tools will
be in /usr/bin, which we check before invoking xcrun. Further, in this
case, dev_tools_path will be /usr/bin, and xctoolchain_path will not
exist, so the fallbacks here are unnecessary.
Move xctoolchain_path to Xcode module, where it belongs
Xcode-only path adjustments should be the same in both environments
Rename Xcode.folder to MacOS.active_developer_dir

This reflects what Apple calls it in more recent versions of Xcode/OS X.
Remove MacOS.dev_tools_path and MacOS.dev_tools_prefix

These methods are no longer required.
TODO