Switch to mod.conf, delete deprecated depends.txt

mods/doors/depends.txt

@@ -1,2 +0,0 @@
-default
-screwdriver?

mods/doors/mod.conf

@@ -0,0 +1,4 @@
+name = doors
+description = Minetest Game mod: doors
+depends = default
+optional_depends = screwdriver