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-dofetch.sh

@@ -43,7 +43,6 @@ if declare -f do_fetch >/dev/null; then
     cd ${XBPS_BUILDDIR}
     [ -n "$build_wrksrc" ] && mkdir -p "$wrksrc"
     run_func do_fetch
-    touch -f $XBPS_FETCH_DONE
 else
     # Run do-fetch hooks.
     run_pkg_hooks "do-fetch"
@@ -57,4 +56,6 @@ fi
 # Run post-fetch hooks.
 run_pkg_hooks post-fetch
 
+touch -f $XBPS_FETCH_DONE
+
 exit 0