expose cgo issue 916

Seems to happen when the main package only has Go files importing "C",
meaning that it has zero "pure Go" files.

To avoid needing two main package Go builds for cgo.txtar,
switch our main package to this scenario as it seems more interesting.

While here, add a test case for a Go callback function taking a C param
as that is relatively common and we had no coverage for it.

This only reproduces the bug; the fix is coming separately.

For #916.

testdata/script/cgo.txtar

@@ -1,6 +1,8 @@
 [!cgo] skip 'this test requires cgo to be enabled'
 
-exec garble build
+! exec garble build
+stderr 'cannot define new methods on non-local type _trieNode'
+stop
 ! stderr 'warning' # check that the C toolchain is happy
 exec ./main
 cmp stdout main.stdout
@@ -33,12 +35,18 @@ go 1.23
 -- main.go --
 package main
 
+// It's important that the main package only has files importing "C",
+// as that used to trigger https://github.com/burrowers/garble/issues/916.
+
+import "C"
+import "test/main/imported"
+
 func main() {
-	regularFunc()
+	imported.RegularFunc()
 	cgoFunc()
 }
--- regular_main.go --
-package main
+-- imported/imported_regular.go --
+package imported
 
 import (
 	"fmt"
@@ -46,7 +54,7 @@ import (
 	"runtime"
 )
 
-func regularFunc() {
+func RegularFunc() {
 	if os.Getenv("GARBLE_TEST_REVERSING") == "true" {
 		_, filename, _, _ := runtime.Caller(0)
 		fmt.Println("regular filename:", filename)
@@ -71,7 +79,7 @@ static int privateAdd(int a, int b) {
 
 extern void goCallback();
 
-static void callGoCallback() {
+static void callGoCallbacks() {
 	goCallback();
 	separateFunction();
 }
@@ -89,7 +97,7 @@ func cgoFunc() {
 	st := C.struct_portedStruct{}
 	fmt.Println(st.PortedField == nil)
 
-	C.callGoCallback()
+	C.callGoCallbacks()
 }
 
 //export goCallback
@@ -101,22 +109,31 @@ func goCallback() {
 		fmt.Println("cgo filename:", filename)
 	}
 }
+
+//export printString
+func printString(cs *C.char) {
+	fmt.Println(C.GoString(cs))
+}
 -- separate.h --
 void separateFunction();
 -- separate.c --
 #include "_cgo_export.h"
+#include <stdio.h>
 
 void separateFunction() {
 	goCallback();
+	printString("string from C");
 }
 -- main.stdout --
 3
 true
 go callback
 go callback
+string from C
 -- reversed.stdout --
-regular filename: test/main/regular_main.go
+regular filename: test/main/imported/imported_regular.go
 3
 true
 go callback
 go callback
+string from C