nix develop: Add --redirect flag to redirect dependencies

This is primarily useful if you're hacking simultaneously on a package
and one of its dependencies. E.g. if you're hacking on Hydra and Nix,
you would start a dev shell for Nix, and then a dev shell for Hydra as
follows:

  $ nix develop \
    --redirect .#hydraJobs.build.x86_64-linux.nix ~/Dev/nix/outputs/out \
    --redirect .#hydraJobs.build.x86_64-linux.nix.dev ~/Dev/nix/outputs/dev

(This assumes hydraJobs.build.x86_64-linux has a passthru.nix
attribute. You can also use a store path.)

This causes all references in the environment to those store paths to
be rewritten to ~/Dev/nix/outputs/{out,dev}. Note: unfortunately, you
may need to set LD_LIBRARY_PATH=~/Dev/nix/outputs/out/lib because
Nixpkgs' ld-wrapper only adds -rpath entries for -L flags that point
to the Nix store.