This patch makes `flutter start` work without a clone of the engine git
repository. Making this work pulled a relatively large refactor of how the
commands interact with application packages and devices. Now commands that want
to interact with application packages or devices inherit from a common base
class that holds stores of those objects as members.
In production, the commands download and connect to devices based on the build
configuration stored on the FlutterCommandRunner. In testing, these fields are
used to mock out the real application package and devices.
Expose the main entry point for the tools via the library lets us run the tools
from the Flutter package, which simplifies the setup for end developers because
they don't need to declare a dependency on sky_tools directly.
