packages: Make 'location' field innate.

* guix/packages.scm (<package>)[location]: Add 'innate' property.
* guix/build-system/gnu.scm (static-package): Remove 'loc' parameter and
  'location' field.
* gnu/packages/autotools.scm (autoconf-wrapper): Remove 'location' field.
* gnu/packages/commencement.scm (gnu-make-boot0, diffutils-boot0, gcc-final):
  Likewise.
* gnu/packages/cross-base.scm (cross): Likewise.
* gnu/packages/emacs.scm (emacs-no-x, emacs-no-x-toolkit): Likewise.
* gnu/packages/make-bootstrap.scm (tarball-package): Likewise.
* gnu/packages/maths.scm (petsc-complex): Likewise.