compiler: cache modules if they are not built yet

vlib/builtin/map.v

@@ -177,6 +177,7 @@ pub fn (m &map) keys() []string {
 }
 
 fn (m map) get(key string, out voidptr) bool {
+	//println('g')
 	if m.root == 0 {
 		return false
 	}

vlib/os/os.v

@@ -333,6 +333,7 @@ pub fn exec(cmd string) ?Result {
 	}
 }
 
+// `system` works like `exec()`, but only returns a return code.
 pub fn system(cmd string) int {
 	mut ret := int(0)
 	$if windows {