blacklist of dependencies is ignored

Original issue: haskell/HTTP#55. The following was run on April 20, 2014 at ~12:00 UTC. I mention this because later uploads to Hackage will likely cause different results.

There is currently a blacklist on Hackage to prevent HTTP < 4000.2.5 from being used. This is respected when installing HTTP directly, but _not_ when HTTP is depended on by another package (in this case, the warp test suite).

Starting with a clean user package database, GHC 7.6.3, and cabal-install 1.16, I get the following:
- `cabal install HTTP --dry-run` indicates that it will install network-2.4.2.3 and HTTP-4000.2.12. Good.
- `cabal install HTTP warp --dry-run` installs those versions too. Again good.
- `cabal install warp --dry-run` uses network-2.5.0.0 and does not include HTTP (HTTP is only used by the test suite). Once again, good.
- `cabal install warp --dry-run --enable-tests` _still_ uses network-2.5.0.0 and HTTP-4000.2.4. _This_ is the problem.

With this bug, Hackage blacklisting can only be relied upon in very narrow cases.

Start by reproducing the issue with the listed cabal install --dry-run commands, especially warp with --enable-tests, using the stated GHC and cabal-install versions if available. Trace dependency resolution for test suites and Hackage blacklists; done means the dry run no longer selects the blacklisted HTTP version.

Written by the indexing model from the issue text.