maint: reinstate LuaRocks init install bug workaround.

Latest LuaRocks still has the bug where Lua source files ending
in `init.lua` are installed to a subdirectory.  Put back the
workaround I removed prematurely.
* lib/std/debug_init.lua: Move from here...
* lib/std/debug_init/init.lua: ...to here.
* local.mk (dist_luastd_DATA): Remove lib/std/debug_init.lua.
(dist_luastddebug_DATA): Add lib/std/debug_init/init.lua.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>