Source support for workspaces

Source plugin implementations should be able to participate in the process of creating a workspace.

This will mostly benefit plugins which use a backing VCS who's upstream should be directed to where BuildStream originally obtained the sources from, instead of having the upstream be BuildStream's internal cache (i.e. have a git checkout's origin be the upstream URL, not the local file:// url in ~/.cache/buildstream)

Related tasks

• Add Source method for initializing the workspace, defaults to Source.stage()
• Support in git plugin
• Support in bzr plugin
• Support in ostree plugin (only useful for pull)