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-doextract.sh

@@ -59,7 +59,6 @@ else
     fi
 fi
 
-touch -f $XBPS_EXTRACT_DONE
 
 [ -d $wrksrc ] && cd $wrksrc
 
@@ -71,4 +70,6 @@ fi
 # Run post-extract hooks
 run_pkg_hooks post-extract
 
+touch -f $XBPS_EXTRACT_DONE
+
 exit 0