How to share modules when using -shared?

[Justin Whear via Digitalmars-d-learn]

I'm trying to build components that I can dynamically link and keep 
running into an issue with sharing modules between the host and the 
pluggable components. Assuming a layout like this:

  host.d  -- loads components at runtime
  a.d     -- a module that builds to `a.so`
  b.d     -- a module that builds to `b.so`
  common.d

If a.d and b.d both import common.d, all is well.  If host.d imports 
common.d as well, I get this at runtime: 
Fatal Error while loading 'a.so':
	The module 'common' is already defined in 'host'.

Test session with sources here: http://pastebin.com/LxsqHhJm

Some of this can be worked around by having host.d use its own extern 
definitions, but how does this work with interfaces?  My tests thus far 
have resulted in object invariant failures.