Have some kind of target API to support multiple Main modules

[snoyberg]

Problem: a single project can contain multiple Main module, specifically: one for each executable, test suite, and benchmark. We'd like to get type information etc for each of those, but can't load them all into GHC at once.

Solution: have an API where the editor can state which of those Main modules is the current target, and then provide information on those files.

[lukehoersten]

+1 just ran into this.

[wraithm]

+1 Also having trouble with this