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.
