Allow seeing include $(x) when there is an include_top comment

Previously, seeing anything other than an interpolate variable
was not allowed.

Bug: 226974242
Test: go test
Change-Id: I48a060f9a3fd19cd67a114d2cb0756ab2be25ce1
diff --git a/mk2rbc/node.go b/mk2rbc/node.go
index 7c39b9e..a01abd8 100644
--- a/mk2rbc/node.go
+++ b/mk2rbc/node.go
@@ -83,7 +83,7 @@
 }
 
 type inheritedDynamicModule struct {
-	path             interpolateExpr
+	path             starlarkExpr
 	candidateModules []*moduleInfo
 	loadAlways       bool
 	location         ErrorLocation
@@ -120,7 +120,7 @@
 }
 
 func (i inheritedDynamicModule) pathExpr() starlarkExpr {
-	return &i.path
+	return i.path
 }
 
 func (i inheritedDynamicModule) needsLoadCheck() bool {