Update artifact push and pull so that they can handle artifact refs

James Ennisrequested to merge
jennis/update_push_pull into master

Description

Now that we can load the build dependencies of artifacts given an artifact ref, I've extended support for bst artifact push so that it can handle artifact refs as arguments.

If a user tries to checkout "all" dependencies of an artifact, BuildStream will error.

This MR partially addresses #822 (closed)

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