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.

Mainly, this adds documentation to members that were previously
lacking documentation.

It also adds a big block of documentation about improving performance
of widgets.

This also removes some references to package:collection and adds
global setEquals and listEquals methods in foundation that we can use.
(setEquals in particular should be much faster than the
package:collection equivalent, though both should be faster as they
avoid allocating new objects.) All remaining references now qualify
the import so we know what our remaining dependencies are.

Also lots of code reordering in Flutter driver to make the code
consistent and apply the style guide more thoroughly.

* Adding first semantics perf test

* review commnts and analyzer fixes

* fix analyzer warning

Well, all the easy ones, anyway.

For some reason `// ignore:` isn't working for me so I've disabled
lints that need that. Also disabled those that require a ton of work
(which I'm doing, but not in this PR, to keep it reviewable).

This adds:
- avoid_init_to_null
- library_names
- package_api_docs
- package_names
- package_prefixed_library_names
- prefer_is_not_empty
- sort_constructors_first
- sort_unnamed_constructors_first
- unnecessary_getters_setters

Turns out VM service only accepts flat parameter name/value pairs. It
only worked this far because vm_service_client (mistakenly) uses JSON
encoder to encode parameters.

This change separates the Command type hierarchy from that of Result and
tightens type constraints on Command object parameters.

This commit contains:

- FlutterDriver API for e2e tests usable in conjunction with package:test
- FlutterDriverExtension to be enabled by the application in order to
  allow an external agent to connect to it and drive user interactions and
  probe into the element tree
- initial implementations of tap, findByValueKey and getText commands (to
  be expanded in future PRs)