This fixes #2 by tweaking `scripts/head.hackage` in two ways:

1. This changes the script to use
   `cabal new-update head.hackage.ghc.haskell.org`, not
   `cabal new-update head.hackage`, since the former is now the name
   of the `repository`. Since `head.hackage.ghc.haskell.org` is
   referred to in quite a few places in the script, I factored this
   out into its own variable.
2. This changes the `url` to use `http://`, not `https://`, so that
   `http-transport: plain-http` works properly. An alternative would
   be to keep the use of `https://` and use `http-transport: curl`
   instead, but that assumes the existence of a `curl` binary on
   one's machine, making it a less portable solution. In any case,
   the use of `http://` in this script shouldn't pose any security
   issues, since `hackage-security` already introduces a security
   layer independent of CAs.

Also turn `head.hackage` into a POSIX sh script

Refactor script and add new `init-local` and `dump-repo` commands.

I'd much prefer to simply specify the path on the command line.