Codechange: change Source into a class with conversion helpers

A Source is either a CompanyID (Headquarters), IndustryID or TownID.
When making those types stronger a lot of casts would be needed, but
with these simple helpers the intent is shown more clearly.

src/cargopacket.cpp

@@ -132,7 +132,7 @@ void CargoPacket::Reduce(uint count)
 /* static */ void CargoPacket::InvalidateAllFrom(Source src)
 {
 	for (CargoPacket *cp : CargoPacket::Iterate()) {
-		if (cp->source == src) cp->source.id = INVALID_SOURCE;
+		if (cp->source == src) cp->source.MakeInvalid();
 	}
 }