xbps-src: refactor build logic into its own script (build.sh).

The build.sh script is now responsible to handle all the logic to build
a source package and its subpackages, as well as all its required
build dependencies. Thanks to this and subshells, dependencies are now
built into its own child process, creating a process tree that can go
nested as long as your system allows forking and has enough memory :-)

This fixes some issues that have been while building pkgs that have lots
of nested dependencies.

common/xbps-src/libexec/xbps-src-prepkg.sh

@@ -45,6 +45,7 @@ fi
 source_file $XBPS_COMMONDIR/environment/build-style/${build_style}.sh
 setup_pkg_depends $pkgname
 run_pkg_hooks pre-pkg
+
 touch -f $XBPS_PREPKG_DONE
 
 exit 0