mojo/discovery: Add Dart generation rule and pubspec

This makes our Dart code into a package.
Until it is published, you can do a dependency_override to get it.

For example, the Croupier project would do this:

name: croupier
  discovery: ">=0.0.0 <0.1.0"
  syncbase: ">=0.0.0 <0.1.0"
  flutter: ">=0.0.3 <0.1.0"
  sky_tools: any
  test: any

    path: ../../mojo/discovery

Note: The .mojom.go file was updated since Mojo has recently changed its
code generator.

Change-Id: I6d311a754a6cbe121d016b2b473cca23d8565142
9 files changed