cabal-install's sdist command fails in the darcs project, with the  
three unwelcome warnings and the error below. A working sdist would  
let you do more isolated builds with cabal-dev, which seems useful.

$ cabal sdist
Distribution quality warnings:
'ghc-options: -O0' is not needed. Use the --disable-optimization  
configure
flag.
'ghc-options: -O2' is rarely needed. Check that it is giving a real  
benefit
and not just imposing longer compile times on your users.
Warning: Cannot run preprocessors. Run 'configure' command first.
Building source dist for darcs-2.5.0.1...
cabal: Error: Could not find module: Version with any suffix:
["gc","chs","hsc","x","y","ly","cpphs","hs","lhs"]

Would be nice to fix

It's not a regression since 2.5, so bumping to 2.10.

I have no issue on my machine with current darcs HEAD:

```
$ cabal sdist
about to hand over
Source tarball created: dist/darcs-2.9.9.tar.gz
$ cabal --version
cabal-install version 1.20.0.3
using version 1.20.0.2 of the Cabal library
```

Still no issue, marking the bug as resolved.