Add a '--list-sources' option for 'cabal sdist'.
Fix the 'index.cache' timestamp check.

Since we're using 'unless' here, we should use (>) instead of (>=).
Refactor D.S.SrcDist.prepareTree.

Separate listing package sources from copying them over. Allows us to remove a
bunch of code from 'D.C.Sandbox.Timestamp'.
Actually implement the '--list-sources' option of 'cabal sdist'.
Create source archives by running 'setup sdist --output-directory'.

Fixes #403.
Don't tell the user about the temporary directory used by sdist.
Use 'sdist --list-sources' for the timestamp check.
Add a lower bound on the test-framework-quickcheck2 dependency.

See batterseapower/test-framework#36.