Change `build-expression->derivation' to support sub-derivation inputs.

* guix/derivations.scm (build-expression->derivation): Change to expect
  INPUTS to have the form (NAME DRV-PATH SUB-DRV) or (NAME DRV-PATH),
  instead of (NAME . DRV-PATH).  Update callers accordingly.

* guix/gnu-build-system.scm, tests/builders.scm, tests/derivations.scm:
  Update accordingly.

Add supporting tools for the GNU Build System.

* guix/derivations.scm (build-expression->derivation): Add all of INPUTS
  as inputs to the final derivation.

* guix/build/gnu-build-system.scm, guix/build/utils.scm,
  guix/gnu-build-system.scm: New files.

* tests/builders.scm ("gnu-build"): New test.

Augment `build-expression->derivation' with #:modules; add `http-fetch'.

* guix/derivations.scm (imported-modules): New procedure.
  (build-expression->derivation): New keyword argument `modules'.
  Use `imported-modules' when MODULES is non-empty, and pass it with
  `-L' to GUILE.

* guix/build/http.scm, guix/http.scm, tests/builders.scm: New files.