* 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.
* 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.
* 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.
