Update artifact checkout to handle artifact refs

James Ennisrequested to merge
jennis/update_checkout into master

Description

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

If a user tries to checkout the runtime dependencies of an artifact, BuildStream will error.

This MR also includes some general cleanup of artifact checkout.

This MR partially addresses #822 (closed)

Edited by James Ennis

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