Current, the `cabal2nix` derivation contains both the executable, and a wrapper
that adds `nix` and `nix-prefetch-scripts`, which are required for some
features.

However, when calling `callCabal2nix` to create a derivation from a cabal file
at evaluation time,
these features are not actually used, but the huge closure of
`nix-prefetch-scripts` (which includes multiple vcs, as well as python and perl)
still needs to be fetched.

This commit splits cabal2nix into a lightweight version that is a standalone
static binary (`cabal2nix-unwrapped`), and a wrapper that includes the proper
dependencies in the path for full usage of the command line
utility (`cabal2nix`).

This commit also switches to the default ghc, to reduce the likelyhood of
building a different ghc when calling `callCabal2nix`.

The tests fail to compile because of a missing module but would
otherwise be fine.

tests need a running mysql instance, so disable them

Signed-off-by: Anders Kaseorg <andersk@mit.edu>