build-system: Rewrite using gexps.

build-system: Rewrite using gexps.

* guix/packages.scm (expand-input): Remove 'store', 'system', and
  'cross-system' parameters; add #:native?.  Rewrite to return
  name/gexp-input tuples.
  (bag->derivation): Adjust accordingly.  Lower (bag-build bag).
  (bag->cross-derivation): Ditto.  Instead of #:native-drvs and
  #:target-drvs, pass #:build-inputs, #:host-inputs, and #:target-inputs.
  (%derivation-cache): Remove.
* gnu/packages/bootstrap.scm (raw-build): Turn into a monadic procedure.
* gnu/packages/commencement.scm (glibc-final)[arguments]: Use
  'gexp-input' for the #:allowed-references argument.
* guix/build-system/cmake.scm (cmake-build): Remove 'store' parameter.
  Switch to the use of gexps and 'gexp->derivation'.
  (lower): Remove #:source from 'private-keywords'.
* guix/build-system/glib-or-gtk.scm (glib-or-gtk-build, lower):
  Likewise.
* guix/build-system/font.scm (font-build): Likewise.
* guix/build-system/gnu.scm (gnu-build): Likewise, and remove
  'canonicalize-reference'.
  (gnu-cross-build): Likewise, and expect #:build-inputs, #:host-inputs,
  and #:target-inputs instead of #:native-drvs and #:target-drvs.
  (lower): Likewise.
* guix/build-system/perl.scm (perl-build, lower): Likewise.
* guix/build-system/python.scm (python-build, lower): Likewise.
* guix/build-system/ruby.scm (ruby-build, lower): Likewise.
* guix/build-system/waf.scm (waf-build, lower): Likewise.
* guix/build-system/trivial.scm (guile-for-build): Remove.
  (trivial-build): Remove 'store' parameter, change to gexps.
  (trivial-cross-build): Ditto, and change to #:build-inputs & co.
* guix/build-system/cargo.scm (cargo-build): Change to 'gexp->derivation'.
* guix/build-system/copy.scm (copy-build): Likewise.
* guix/build-system/dune.scm (dune-build): Likewise.
* guix/build-system/guile.scm (guile-build, guile-cross-build):
  Likewise.
* guix/build-system/meson.scm (meson-build): Likewise.
* guix/build-system/ocaml.scm (ocaml-build): Likewise.
* guix/build-system/scons.scm (scons-build): Likewise.
* guix/build-system/texlive.scm (texlive-build): Likewise.
* guix/build-system/android-ndk.scm (android-ndk-build): Likewise.
* guix/build-system/ant.scm (ant-build): Likewise.
* guix/build-system/asdf.scm (asdf-build/source, asdf-build): Likewise.
* guix/build-system/chicken.scm (chicken-build): Likewise.
* guix/build-system/clojure.scm (clojure-build): Likewise.
(source->output-path, maybe-guile->guile): Remove.
* guix/build-system/dub.scm (dub-build): Likewise.
* guix/build-system/emacs.scm (emacs-build): Likewise.
* guix/build-system/go.scm (go-build): Likewise.
* guix/build-system/haskell.scm (haskell-build): Likewise.
* guix/build-system/julia.scm (julia-build): Likewise.
* guix/build-system/linux-module.scm (linux-module-build)
(linux-module-build-cross): Likewise.
* guix/build-system/maven.scm (maven-build): Likewise.
* guix/build-system/minify.scm (minify-build): Likewise.
* guix/build-system/node.scm (node-build): Likewise.
* guix/build-system/qt.scm (qt-build, qt-cross-build): Likewise.
* guix/build-system/r.scm (r-build): Likewise.
* guix/build-system/rakudo.scm (rakudo-build): Likewise.
* guix/build-system/renpy.scm (renpy-build): Likewise.
* tests/builders.scm ("gnu-build"): Call 'store-lower' on 'gnu-build'.
  Pass #:source parameter.
* tests/packages.scm ("search paths"): Use 'abort-to-prompt' instead of
  a normal return from the 'build' method.
  ("package->bag, sensitivity to %current-target-system"): Change 'build'
  to match the new build system signature.

squash! build-system: Rewrite using gexps.

squash! build-system: Rewrite using gexps.