ssbc: Implement workspace resolver in backend

Created by: eseliger

This PR pulls in two pieces of code from src-cli:

• FindDirectoriesInRepos
• findWorkspaces and their test suites. We're actually not doing much magic here, rename a couple of methods to return workspaces and not repoRevs, slightly change the GraphQL schema to expose the new information we have at hand now and let the frontend make use of that data.