* guix/derivations.scm (derivation->output-path,
derivation->output-paths): New procedures.
(derivation-path->output-path): Use 'derivation->output-path'.
(derivation-path->output-paths): Use 'derivation->output-paths'.
(derivation): Accept 'derivation?' objects as inputs. Return a single
value.
(build-derivations): New procedure.
(compiled-modules): Use 'derivation->output-paths'.
(build-expression->derivation)[source-path]: Add case for when the
input matches 'derivation?'.
[prologue]: Accept 'derivation?' objects in INPUTS.
[mod-dir, go-dir]: Use 'derivation->output-path'.
* guix/download.scm (url-fetch): Adjust to the single-value return.
* guix/packages.scm (package-output): Use 'derivation->output-path'.
* guix/scripts/build.scm (guix-build): When the argument is
'derivation-path?', pass it through 'read-derivation'.
Use 'derivation-file-name' to print out the .drv file names, and to
register them. Use 'derivation->output-path' instead of
'derivation-path->output-path'.
* guix/scripts/package.scm (roll-back): Adjust to the single-value
return.
(guix-package): Use 'derivation->output-path'.
* guix/ui.scm (show-what-to-build): Adjust to deal with 'derivation?'
objects instead of .drv file names.
* gnu/system/grub.scm (grub-configuration-file): Use
'derivation->output-path' instead of 'derivation-path->output-path'.
* gnu/system/vm.scm (qemu-image, system-qemu-image): Likewise.
* tests/builders.scm, tests/derivations.scm, tests/packages.scm,
tests/store.scm, tests/union.scm: Adjust to the new calling
convention.
* doc/guix.texi (Defining Packages, The Store, Derivations): Adjust
accordingly.
