Allow pulling build scope of artifacts which are not cached locally

In any case that the user has requested artifacts be pulled, and that artifact targets have been specified, pre-emptively pull the target artifact metadata so that the operation can complete coherently.

Specifically, this fixes:

• bst artifact pull --deps build <artifact name>
• bst artifact checkout --deps build --pull <artifact name>

In these cases, we cannot know the build dependencies of the artifact if we do not at least try to obtain the toplevel target artfiact(s) first, so we instate a policy to initially download metadata.

Currently this unfortunately downloads the entire artifact, but should be optimized later on to download only the required metadata, added FIXME comment around here.

added 4 commits

• 57293c8b - _stream.py: Pre-emptive pulling of artifact metadata in some cases
• 11e495b7 - _stream.py: Added new _reset() function.
• b17e3dc8 - tests/frontend/artifact_pull.py: Test pulling artifacts with various deps options
• da492879 - tests/frontend/artifact_checkout.py: Test checking out remote artifacts

Compare with previous version

changed the description