This allows the scheduler library to depend on the services library
and the painting library to depend on the scheduler library without
the services library having to depend on the scheduler library.
While I was at it I also cleaned up some of the binding logic: the
licenses logic can now be overridden (and the test library does so),
and the image cache can now be overridden as well.
This patch reorganises flutter_driver's lib/src directory into three
subdirectories:
* driver: sources transitively imported/exported by flutter_driver.dart,
which defines the API used by driver tests executed on the host machine.
* extension: sources transitively imported/exported by
driver_extension.dart, which defines the API used to instrument a
Flutter app running on the target device for use with a driver test.
* common: sources common to both driver and extension code.
This makes rolling Flutter into repositories that use Bazel (or
Bazel-like build systems) significantly more robust, due to the simpler
glob patterns involved.
