Commit WIP code splitting functionality to a few independent shell scripts.

Much of the code from shutils has been merged into new scripts in libexec.
shutils/build_dependencies.sh: s/KEEP_AUTODEPS/XBPS_KEEP_ALL/
xbps-src.sh.in: fix PATH in no-chroot case.
Multiple fixes and improvements. Building from scratch with nested deps works.
Added XBPS_DEBUG_PKGS option (disabled) to enable building -dbg subpkgs.
Do not propagate fulldepends to the target pkg, return early when all deps are built.
xbps-src.sh.in: run set_cross_defvars after required vars are set up.
xbps-src.sh.in: fix bootstrap-update.
Commit WIP support for cross builds; host and target deps are built from scratch.

But it's not perfect and needs more tweaks/fixes.
Set a working PATH for xbps-src <target> in chroot to work.
shutils/build_dependencies.sh: pass correct var for cross builds.
shutils/common.sh: set a value to  if unset in remove_autodeps.
Multiple fixes, improvements and massive speedups.
shutils/chroot.sh: propagate XBPS_DEBUG_PKGS into the chroot.
shutils/common.sh: fix remove_autodeps when not building any pkg.
Implement a template format merging subpkg templates into main pkg.

Every generated pkg now must be explicitly defined in a function "<pkgname>_package".
In that function a pkg_install() function is required to explicitly "move"
package files from "main pkg".

The template.options files are now also obsolete (and its do_options() func);
package options should be specified in the package template.

The "subpackages" var is now obsolete. The package list is generated automatically
and sorted alphabetically.
shutils/common.sh: fix a msg.
Fixed creation of -dbg packages.
libexec/xbps-src-dostrip.sh: error out if PKGDESTDIR doesn't exist.
libexec/xbps-src-genrdeps.sh: error out if PKGDESTDIR doesn't exist.