All invocations should go via the IMobileDevice class in mac.dart.

Extract out IMobileDevice class, move class to idevice_id, ideviceinfo
(and eventually other libimobiledevice tools such as iproxy) behind this
interface.

Add tests for the case where libimobiledevice is not installed, the case
where it returns no devices, and the case where it returns device IDs.

Does not yet migrate hasIdeviceId since that results in a cascade of
breaking interface changes that's significant enough for a separate
patch.

Ensure that flutter doctor returns a partial installation status when
ideviceid is not installed.

* Don’t automatically pod setup if the user never did it

* fix/add test

* rename getters

(follow-on to #8698)

`all.dart` is no longer needed. Furthermore, it causes tests to
be skipped, or to silently fail to run anything.

Fixes #7941

* 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).

Ref: https://developer.apple.com/xcode/