Cant find exported type when ppx is used

Hi @jaredly! I was trying to use milk and https://github.com/ELLIOTTCABLE/bs-deriving

when i have ppx in bsconfig.json milk throws error that it can't find desired type.
```
Fatal error: exception Failure("No exported type t in module file:///~/demo/src/Config.re ")
Raised at file "stdlib.ml", line 33, characters 17-33
Called from file "belt/belt_List.ml", line 172, characters 29-34
Called from file "src/Milk.re", line 66, characters 4-675
Called from file "src/Milk.re", line 139, characters 47-66
Called from file "bin/Main.re", line 104, characters 4-47
```

removing ppx from bsconfig helps

Start by reproducing the failure with ppx enabled in bsconfig.json and the Config.re example, then inspect the type lookup paths in src/Milk.re and the reported call site in bin/Main.re. Done means Milk can find the exported type when the bs-deriving ppx is configured, without the reported exception.

Written by the indexing model from the issue text.