Allow derivations with input derivations.

Allow derivations with input derivations.

* guix/derivations.scm (derivation-path->output-path): New procedure.
  (derivation-hash): Call `memoize'.  In the fixed-output case, convert
  HASH-ALGO to a string.  In the other case, sort inputs in the
  alphabetical order of their hex hash.  For inputs with no sub-drvs,
  add "out" as the sub-drv.

* guix/utils.scm (%nixpkgs-directory): New parameter.
  (nixpkgs-derivation, memoize): New procedures.

* tests/derivations.scm ("build derivation with 1 source"): Remove
  useless shebang.
  (%coreutils): New variable.
  ("build derivation with coreutils"): New test.