fix: external_module.go with sinks/sources build failures (#1132)

Fixes #1095

Sink & Source verbs would have a schema like
```
verb returnsUser(Unit) two.UserResponse
```
but the scaffolded `external_module.go` version of it would be:
```
func ReturnsUser(ctx context.Context) (UserResponse, error)
```

When generating imports, we were finding `schema.Unit` in the verb which
triggered an inclusion of ftl
This caused the exernal module to fail to build because ftl was not
being used by the generated code.

This fix now skips any unit in the schema if the parent is a verb

backend/schema/visit.go

@@ -19,6 +19,12 @@ func VisitExcludingMetadataChildren(n Node, visit func(n Node, next func() error
 	return visit(n, func() error {
 		if _, ok := n.(Metadata); !ok {
 			for _, child := range n.schemaChildren() {
+				_, isParentVerb := n.(*Verb)
+				_, isChildUnit := child.(*Unit)
+				if isParentVerb && isChildUnit {
+					// Skip visiting children of a verb that are units as the scaffolded code will not inclue them
+					continue
+				}
 				if err := VisitExcludingMetadataChildren(child, visit); err != nil {
 					return err
 				}

buildengine/build_go_test.go

@@ -67,7 +67,6 @@ package other
 
 import (
   "context"
-  "github.com/TBD54566975/ftl/go-runtime/ftl"
 )
 
 var _ = context.Background

go-runtime/compile/schema_test.go

@@ -148,7 +148,7 @@ func TestExtractModuleSchemaTwo(t *testing.T) {
 		verb callsTwo(two.Payload<String>) two.Payload<String>  
 			+calls two.two
 	  
-		verb returnsUser(Unit) two.UserResponse?
+		verb returnsUser(Unit) two.UserResponse
 	  
 		verb two(two.Payload<String>) two.Payload<String>
 	  }

go-runtime/compile/testdata/two/two.go

@@ -42,10 +42,10 @@ func CallsTwo(ctx context.Context, req Payload[string]) (Payload[string], error)
 }
 
 //ftl:export
-func ReturnsUser(ctx context.Context) (ftl.Option[UserResponse], error) {
-	return ftl.Some[UserResponse](UserResponse{
+func ReturnsUser(ctx context.Context) (UserResponse, error) {
+	return UserResponse{
 		User: User{
 			Name: "John Doe",
 		},
-	}), nil
+	}, nil
 }