Artifacts from local engine builds will be found based on the --local-engine flag

Host tools can be found in the artifact cache directory for the host platform.
If a developer wants to use a local engine build instead, then provide an
--engine-build flag that selects the specific engine build variant.

Update flutter_tools for the "unoptimized" engine build flag and the new output directory naming scheme (#3832)

* fix analysis errors

* review comments; fix test

* re-add an export for debugPrint

Also update the engine to a build where these tools have been uplaoded

* rework flutter run

* fix npe with --debug-port

* connect to obs and exit when that conneciton closes

* update todos

This is currently hidden for development use.  It will select artifacts from
the android_{Debug,Release}_Deploy output in a local engine build.

* rename the --develop option to --debug

* fail if both --debug and --deploy are specified

* download android-arm-deploy

* wire up --deploy to android-arm-deploy

* fix interpolation

* use the new cache code to locate more artifacts

* add todo

* review comments

The .packages file is replacing the packages directory of symlinks.

Fixes #1942.

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

This flag builds a local FLX file and pushes that to the device instead of
using an HTTP server.

Instead of calling through `pub` to invoke build, this patch refactors the
build command so that it can be called directly.