1) Moved basic utility code into base/ directory to make it clear which code
doesn't depend on Flutter-specific knowldge.
2) Move the CommandRunner subclasses into a runner/ directory because these
aren't commands themselves.
Previously, we assumed the first build configuration would have one. Now we
keep looking until we find one. Also, re-ordered the configurations so that
you'll get the Android one if you have both, which is probably what you would
expect.
Fixes#100
