Refactor icon export to be based on prototypes

Use the defines.prototypes data structure from the game to find which
prototype sub-types belong in the same base type, and export icons based
on these base type.

This renames the planets category to space-location, signal to
virtual-signal and plits item up into item and fluid.  And changes the
exported metadata to include prototypes with no icons, as future code
might want to use it to for example enumerate all item prototypes.

packages/host/lua/export/control.lua

@@ -9,4 +9,5 @@ script.on_init(function()
 	for name, mod_setting in pairs(mod_settings) do
 		send_json("mod_setting_mod", { name = name, mod = mod_setting.mod })
 	end
+	send_json("defines", defines)
 end)