Cannot write lookup referring to another workspace

To build packages in a workspace you currently need to maintain a set of build environments in the same workspace.

Our lookup syntax is currently specific to a single workspace, with no mechanism to refer to a collection in another workspace.

e.g in a private workspace on debusine.freexian.com:

detail: 'Task data: {''input'': {''source_artifact'': 5043}, ''backend'': ''unshare'', ''environment'': ''freexian/match:format=tarball:codename=buster:architecture=amd64:backend=unshare'', ''host_architecture'': ''amd64'', ''build_components'': [''all'', ''any'']} Error: "''freexian'' does not exist or is hidden"'