write a packages.cfg when globally installing a package

[sethladd]

In order to properly find dependencies, and thus their resources, please write a packages.cfg file next to the snapshot generated during pub global activate

Thanks!

[nex3]

Maybe this was the intent of the issue, but to be clear: I want to wait on this until the resource-loading story has a concrete implementation in the VM. It doesn't make sense to implement pub support for something that doesn't exist yet and whose conventions may still change.

[DartBot]

This issue has been moved to dart-lang/pub#22.