Make internals of Lisp_Package visible to Lisp

Make internals of Lisp_Package visible to Lisp

Done to be able to do as much as possible from Lisp.

* src/lisp.h (Lisp_Package): Rename used_packages to use_list.
* src/pkg.c: Rename used_packages to use_list.
(Fpackage_percent_name): (Fpackage_percent_set_name):
(Fpackage_percent_nicknames):
(Fpackage_percent_set_nicknames):
(Fpackage_percent_use_list):
(Fpackage_percent_set_use_list):
(Fpackage_percent_shadowing_symbols):
(Fpackage_percent_set_shadowing_symbols):
(Fpackage_percent_symbols): New functions for getting and setting
internals of Lisp_Package.
(syms_of_pkg): defsubr them.