Add `bst source` command group

Background

See https://mail.gnome.org/archives/buildstream-list/2018-November/msg00106.html and its replies for full context. The summary is that similar to how we plan on having a bst artifact command group to handle all operations related to artifacts, it would make sense to have a bst source command group to handle all source operations.

Task description

• Add new source command group
• Move fetch, track and source-checkout commands under the new group
• Mark fetch and track commands as obsolete

Acceptance Criteria

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