Ignore variable assignments that come after a rule definition

These private variables sometimes have references to make
variables like $< or $@. When converted to starlark, they
become invalid local variables like _< and _@, preventing
the file from being loaded even if it's never executed.

Bug: 226974242
Test: go test
Change-Id: Iafd4c6939731f3b7c051c9e41464134d5b672f23

mk2rbc/mk2rbc.go

@@ -531,7 +531,7 @@ func (ctx *parseContext) backNode() {
 
 func (ctx *parseContext) handleAssignment(a *mkparser.Assignment) []starlarkNode {
 	// Handle only simple variables
-	if !a.Name.Const() {
+	if !a.Name.Const() || a.Target != nil {
 		return []starlarkNode{ctx.newBadNode(a, "Only simple variables are handled")}
 	}
 	name := a.Name.Strings[0]