Also update tests to be compatible with the presence of iOS and add tests for list and install.

Add listen command and basic test, and don’t do unnecessary repeated work when listening or poking the android server.

Also add unchecked runSync wrapper and use it in places where command failures don’t matter.

Also refactors the command line assembling to make a bit more sense.

Fixes #53

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.

The flags control which build of mojo to run (Debug or Release).
Default is Release.

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

sky_viewer.mojo no longer needs a window manager embedding as of
sky_engine 0.0.27, so this updates the sky_tools run_mojo command so it works
again. This really should be expressed in the pubspec dependencies to avoid
broken combinations but that needs to happen on the sky side.

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.

This teaches commands that need binary artifacts to explicitly instantiate an
instance of the ArtifactStore with the appropriate packageRoot string. The
ArtifactStore can then remember the package root and compute the engine
revision when created and remember those for subsequence calls.

This teaches sky_tools mojo_run --android to invoke mojo's devtool's mojo_run
script with the right flags for invoking sky_viewer on android. This tells the
devtools script to load sky_viewer.mojo from https://storage.googleapis.com/...
and to load app.flx (or whatever the developer specifies as --app) from the
filesystem using the devtools http server.

`dart bin/sky_tools.dart --debug --sky-src-path=/path/to/sky/src/ install` now works.

This initial version assumes the developer has mojo_shell and all other services
sitting on disk somewhere and that they're on linux and only want to run on
linux. This can be generalized down the line to support more use cases. This
downloads the sky_viewer.mojo corresponding to the packages/sky_engine/REVISION
in the developer's directory, so they can specify whatever revision they want.
sky_tools run_mojo downloads sky_viewer.mojo into its cache directory if it is
not present and constructs a command line to pass to mojo_shell that maps the
shebang stamped into the flx to the downloaded sky_viewer.mojo.

Since sky_viewer.mojo lives in the cloud and mojo_shell can load from the cloud
this could also map to an https URL. This should likely be an option.