Path to build artifacts changes depending on the build-type

For example, if I have a package `foo` with an executable `exe`, with `build-type: Simple` the executable will be under

```
dist-newstyle/build/x86_64-linux/ghc-8.2.1/foo-1.0/c/exe/build/exe/exe
```

But if I change the build type to `Custom`, it will be under

```
dist-newstyle/build/x86_64-linux/ghc-8.2.1/foo-1.0/build/exe/exe
```

This is with Cabal 2.0.

/cc @ezyang