eden/edenapi and mononoke integration tests: add edenapi/tools to getdeps and use them in tests (#51)

Summary:
Pull Request resolved: https://github.com/facebookexperimental/eden/pull/51

This diff extends capabilities of CargoBuilder in getdeps so that individual manifests can be build even without workspaces. Thanks to that a build for edenapi/tools can be made and its artifacts can be used in mononoke integration tests.

Reviewed By: StanislavGlebik

Differential Revision: D23574887

fbshipit-source-id: 8a974a6b5235d36a44fe082aad55cd380d84dd09