Move Source-repository out of Library section to prevent warning

This patch fixes the warnings we see in `cabal configure`:

```
% cabal configure
Warning: monad-par.cabal: Unexpected section 'source-repository' on line 89
Resolving dependencies...
Warning: monad-par.cabal: Unexpected section 'source-repository' on line 89
Configuring monad-par-0.3.4.6...
```