Support `main-is` inside "common stanzas"

My use case is that I have a lot of executables placed in different folders. Every executable contains a single Haskell source code file called `Main.is`. I would like to remove some boilerplate by putting `main-is: Main.hs` into `common` stanza (with other options). However, when I do this, I see the following build error message:

```
No 'main-is' field found for executable my-exe1
No 'main-is' field found for executable my-exe2
No 'main-is' field found for executable my-exe3
No 'main-is' field found for executable my-exe4
No 'main-is' field found for executable my-exe5
```

It would be really nice if common stanzas could support `main-is` field as well.