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

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)

approved this merge request

resolved all threads

mentioned in issue #1113

added 12 commits

• b9599998...67f1dff4 - 10 commits from branch master
• 5937609a - cli.py: Allow push to handle artifact refs
• 8b40905d - cli.py: Allow pull to handle artifact refs

Compare with previous version

added 57 commits

• 8b40905d...1568268d - 55 commits from branch master
• d787d664 - cli.py: Allow push to handle artifact refs
• 0c7618a4 - cli.py: Allow pull to handle artifact refs

Compare with previous version

added 8 commits

• 0c7618a4...d305c5b6 - 6 commits from branch master
• fa88503a - cli.py: Allow push to handle artifact refs
• 661a290f - cli.py: Allow pull to handle artifact refs

Compare with previous version

added 2 commits

• 95d3334f - _project.py: Don't only load cached artifacts
• 821fd896 - cli.py: Allow pull to handle artifact refs

Compare with previous version

resolved all threads

approved this merge request

assigned to @marge-bot123

added 16 commits

• 821fd896...d3cb30ce - 13 commits from branch master
• 5529b914 - cli.py: Allow push to handle artifact refs
• 1cd7a829 - _project.py: Don't only load cached artifacts
• cdb698f3 - cli.py: Allow pull to handle artifact refs

Compare with previous version

merged

mentioned in commit 3513a80d