DHE: Workspace dependencies

[bmingles]

This is the DHE counterpart of DHC: Workspace dependencies #104. Same goals apply, key difference being that DHE servers are remote and will have no direct access to files in the local workspace. We may be able to leverage import hooks to address this:

• https://stackoverflow.com/questions/43571737/how-to-implement-an-import-hook-that-can-modify-the-source-code-on-the-fly-using
• https://pypi.org/project/importhook/