cmd/go: output missing GoDebug entries

The `go help mod edit` command references the GoDebug struct, but `go
mod edit -json` was not displaying them when appropriate.

Fixes #75105

Change-Id: Iec987882941e01b0cf4d4fe31dda9e7a6e2dde87
Reviewed-on: https://go-review.googlesource.com/c/go/+/706757
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>

src/cmd/go/internal/modcmd/edit.go

@@ -586,6 +586,7 @@ type fileJSON struct {
 	Module    editModuleJSON
 	Go        string      `json:",omitempty"`
 	Toolchain string      `json:",omitempty"`
+	GoDebug   []debugJSON `json:",omitempty"`
 	Require   []requireJSON
 	Exclude   []module.Version
 	Replace   []replaceJSON
@@ -599,6 +600,11 @@ type editModuleJSON struct {
 	Deprecated string `json:",omitempty"`
 }
 
+type debugJSON struct {
+	Key   string
+	Value string
+}
+
 type requireJSON struct {
 	Path     string
 	Version  string `json:",omitempty"`
@@ -657,6 +663,9 @@ func editPrintJSON(modFile *modfile.File) {
 	for _, i := range modFile.Ignore {
 		f.Ignore = append(f.Ignore, ignoreJSON{i.Path})
 	}
+	for _, d := range modFile.Godebug {
+		f.GoDebug = append(f.GoDebug, debugJSON{d.Key, d.Value})
+	}
 	data, err := json.MarshalIndent(&f, "", "\t")
 	if err != nil {
 		base.Fatalf("go: internal error: %v", err)

src/cmd/go/testdata/script/mod_edit_issue75105.txt

@@ -0,0 +1,36 @@
+env GO111MODULE=on
+
+go mod edit -godebug 'http2debug=2'
+cmp go.mod go.mod.edit.want1
+
+go mod edit -json
+cmp stdout go.mod.edit.want2
+-- go.mod --
+module foo
+
+go 1.25.0
+-- go.mod.edit.want1 --
+module foo
+
+go 1.25.0
+
+godebug http2debug=2
+-- go.mod.edit.want2 --
+{
+	"Module": {
+		"Path": "foo"
+	},
+	"Go": "1.25.0",
+	"GoDebug": [
+		{
+			"Key": "http2debug",
+			"Value": "2"
+		}
+	],
+	"Require": null,
+	"Exclude": null,
+	"Replace": null,
+	"Retract": null,
+	"Tool": null,
+	"Ignore": null
+}