`cabal sdist` concatenates `main-is`

consider the fragment

```
if impl(ghcjs)
main-is: JSMain.hs
else
main-is: Main.hs
```

With cabal 1.24 this causes `cabal sdist` to fail with

```
Building source dist for foo-0.1.0.0...
cabal: JSMain.hsMain.hs doesn't exist
```

@dcoutts says

> this is a bug in the `sdist` code, it's using `flattenPackageDescription` to get both sides of all conditionals to find all files but that doesn't work for `main-is` apparently

/cc @ezyang