* guix/build-system.scm (<build-system>)[build, cross-build]: Remove.
[lower]: New field.
(<bag>): New record type.
(make-bag): New procedure.
* guix/packages.scm (bag-transitive-inputs, bag-transitive-build-inputs,
bag-transitive-host-inputs, bag-transitive-target-inputs,
package->bag): New procedures.
(package-derivation): Use it; use the bag, apply its build procedure,
etc.
(package-cross-derivation): Likewise.
* gnu/packages/bootstrap.scm (raw-build, make-raw-bag): New procedure.
(%bootstrap-guile): Use them.
* guix/build-system/trivial.scm (lower): New procedure.
(trivial-build, trivial-cross-build): Remove 'source' parameter. Pass
INPUTS as is.
(trivial-build-system): Adjust accordingly.
* guix/build-system/gnu.scm (%store, inputs-search-paths,
standard-search-paths, expand-inputs, standard-inputs): Remove.
(gnu-lower): New procedure.
(gnu-build): Remove 'source' and #:implicit-inputs? parameters.
Remove 'implicit-inputs' and 'implicit-search-paths' variables. Get
the source from INPUT-DRVS.
(gnu-cross-build): Likewise.
(standard-cross-packages): Remove call to 'standard-packages'.
(standard-cross-inputs, standard-cross-search-paths): Remove.
(gnu-build-system): Remove 'build' and 'cross-build'; add 'lower'.
* guix/build-system/cmake.scm (lower): New procedure.
(cmake-build): Remove 'source' and #:cmake parameters. Use INPUTS and
SEARCH-PATHS as is. Get the source from INPUTS.
* guix/build-system/perl.scm: Likewise.
* guix/build-system/python.scm: Likewise.
* guix/build-system/ruby.scm: Likewise.
* gnu/packages/cross-base.scm (cross-gcc): Change "cross-linux-headers"
to "linux-headers".
(cross-libc)[xlinux-headers]: Pass #:implicit-cross-inputs? #f.
Likewise. In 'propagated-inputs', change "cross-linux-headers" to
"linux-headers".
* guix/git-download.scm (git-fetch): Use 'standard-packages' instead of
'standard-inputs'.
* tests/builders.scm ("gnu-build-system"): Remove use of
'build-system-builder'.
("gnu-build"): Remove 'source' and #:implicit-inputs? arguments to
'gnu-build'.
* tests/packages.scm ("search paths"): Adjust to new build system API.
("package-cross-derivation, no cross builder"): Likewise.
* doc/guix.texi (Build Systems): Add paragraph on bags.
