* [mtouch/mmp] Only use stdout from xcrun as the path of the file that was found. Fixes#8147.
xcrun can print stuff to stderr even if it succeeds and prints what it found
to stdout, which means we have to separate stdout and stderr when capturing
xcrun's output.
This also means that we need a few changes to our RunCommand API:
* Add support for capturing stdout and stderr separately by providing
different callbacks/StringBuilder instances.
* Use overloads instead of default arguments, because with default arguments
we end up with ambiguous matches in a lot of places with the new overloads.
Fixes https://github.com/xamarin/xamarin-macios/issues/8147.
* [tests] Fix mmptest build.
* [mtouch/mmp] Fix process log output.
c6e9932be9