Add (guix describe) and use it to initialize '%package-search-path'.

* guix/describe.scm: New file.
* Makefile.am (MODULES): Add it.
* gnu/packages.scm (%default-package-module-path): New variable.
(%package-module-path): Honor 'package-path-entries'.
* build-aux/update-NEWS.scm (main): Use %DEFAULT-PACKAGE-MODULE-PATH
instead of (last (%package-module-path)).