fix fieldToStruct for generics instantiated across package boundaries (#1028)

When compiling a package that instantiates a generic type from another
package, info.Types only contains the instantiated *types.Named. The
*types.Struct case in recordFieldToStruct returns early on instantiated
structs (field != field.Origin()), so the origin struct was never
recorded in fieldToStruct. At rewrite time, looking up the origin field
then failed with "could not find struct for field X".

Fix by also recursing into typ.Origin() for instantiated named types,
so the uninstantiated struct is visited and its fields are recorded.

Fixes #1027.

testdata/script/typeparams.txtar

@@ -7,6 +7,15 @@ go 1.23
 -- garble_main.go --
 package main
 
+import "test/main/lib"
+
+type Blob []byte
+
+func (b Blob) MarshalBinary() ([]byte, error)  { return b, nil }
+func (b *Blob) UnmarshalBinary(d []byte) error { *b = append((*b)[:0], d...); return nil }
+
+type Name string
+
 func main() {
 	GenericFunc[int, int](1, 2)
 	var _ GenericVector[int]
@@ -21,6 +30,11 @@ func main() {
 	ga.list = nil
 	var gan genericAliasNamed
 	gan.list = nil
+
+	e := lib.Entry[Name, Blob, *Blob]{Key: "k", Data: Blob("v")}
+	r := lib.Load[Name, Blob, *Blob](e)
+	_ = r.Data.Key
+	_ = r.Data.Value
 }
 
 func GenericFunc[GenericParamA, B any](x GenericParamA, y B) {}
@@ -97,3 +111,34 @@ func byKeys[T any](m map[string]T) []struct {
 }
 
 var _ = byKeys(map[string]int{"one": 1})
+-- lib/lib.go --
+package lib
+
+import "encoding"
+
+type Pair[Key, Value any] struct {
+	Key   Key
+	Value Value
+}
+
+type Result[TData any] struct {
+	Data TData
+	Err  error
+}
+
+type Entry[TKey ~string, TData encoding.BinaryMarshaler, TPtr interface {
+	*TData
+	encoding.BinaryUnmarshaler
+}] struct {
+	Key  TKey
+	Data TData
+}
+
+func Load[TKey ~string, TData encoding.BinaryMarshaler, TPtr interface {
+	*TData
+	encoding.BinaryUnmarshaler
+}](e Entry[TKey, TData, TPtr]) Result[Pair[TKey, TData]] {
+	return Result[Pair[TKey, TData]]{
+		Data: Pair[TKey, TData]{Key: e.Key, Value: e.Data},
+	}
+}

transformer.go

@@ -137,7 +137,7 @@ func recordFieldToStruct(typ types.Type, done map[*types.Named]bool, fieldToStru
 			return
 		}
 		done[typ] = true
-		recordFieldToStruct(typ.Underlying(), done, fieldToStruct)
+		recordFieldToStruct(typ.Origin().Underlying(), done, fieldToStruct)
 	case *types.Struct:
 		for field := range typ.Fields() {
 			if field != field.Origin() {