Debugging simple library (not an executable)

[alt-romes]

When one debugs a library rather than an executable we run into a bug whose error message is similar to #79 about duplicate libraries.
(i.e. when we set entryFile: MyLib.hs, entryPoint: )

Matthew says the problem is the added "special target" near line 88 in GHC.Debugger.Session

[alt-romes]

I've fixed this in the branch for #47 by having if null targets then [special_target] else targets in the targets we load.

Make sure to add a test before closing!