`cabal sdist` fails to collect all `main-is`

Trying to workaround #3313 I exposed a different bug:

instead of making `main-is` conditional, now I made `hs-source-dirs` conditional:

```
main-is: Main.hs
if impl(ghcjs)
hs-source-dirs: src-jsexe
else
hs-source-dirs: src-exe
```

however, `cabal sdist` resulted in a source-tarball containing only the `src-jsexe/Main.hs` but missing the `src-exe/Main.hs` file

(PS: a working workaround here is to use `extra-source-files: src-exe/Main.hs`)