Revert "guix build: Autoload (gnu packages)."

This reverts commit fd62b4cf88, which
would lead 'GUIX_PACKAGE_PATH' to be ignored for instance when using
'guix build -f file.scm', as shown by 'tests/guix-build.sh'.

guix/scripts/build.scm

@@ -46,9 +46,7 @@
   #:use-module (srfi srfi-34)
   #:use-module (srfi srfi-35)
   #:use-module (srfi srfi-37)
-  #:autoload   (gnu packages) (%package-module-path
-                               %patch-path
-                               specification->package)
+  #:use-module (gnu packages)
   #:use-module ((guix status) #:select (with-status-verbosity))
   #:use-module ((guix progress) #:select (current-terminal-columns))
   #:use-module ((guix build syscalls) #:select (terminal-columns))