* Fix potential NPE in iOS doctor check
In case Xcode is not installed, the xcode-select path may be null.
* fixup! Fix potential NPE in iOS doctor check
If the developer has only installed the Xcode command-line tools,
xcode-select and some other tools may be present, but xcodebuild will be
missing. In this case, output a slightly improved message indicating
that the installation is incomplete rather than missing.
* Add doctor check for Python 'six' module
Required as part of Xcode lldb module. In all likelihood, if we
encounter this situation, the developer is using a custom Python install
(e.g., via MacPorts or Homebrew).