* 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

Related to #3608

* 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

* better messaging about windows support

* fix lints

* have flutter precache print if there's no work to do

* tweak precache

* move the engine download code to dart

* fix populating the bin/cache/pkg dir

* add cache.dart to help manage the cache dir

* sp

I'm trying to get a feel for the code by writing some simple cleanup patches.

This makes the 'package-root' option universal for sky_tools and configures the
ArtifactStore with it statically at startup. The actual sky_engine revision
is computed on demand.

Refactor all the commands to be Commands from the Args package.  Also use CommandRunner for the top-level command.

This adds the following commands to sky_tools:
  sky_tools cache clear: Nukes all local artifacts in the cache
  sky_tools cache populate: Populates the cache with all known artifacts

This is useful both to fix busted caches and to make sure that the cache is
fully populated so that subsequent operations can proceed without needing
network access.