cmd/go/internal/modcmd: fix go mod edit -module replaces empty string

Fixes golang/go#28820.

Change-Id: Id931617efcf161ec934eb6d44062ad95e8a6ab8d
Reviewed-on: https://go-review.googlesource.com/c/150277
Run-TryBot: Baokun Lee <nototon@gmail.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

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

@@ -183,7 +183,7 @@ func runEdit(cmd *base.Command, args []string) {
 	}
 
 	if *editModule != "" {
-		modFile.AddModuleStmt(modload.CmdModModule)
+		modFile.AddModuleStmt(*editModule)
 	}
 
 	if *editGo != "" {

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

@@ -42,6 +42,12 @@ go mod edit -fmt # without -print, should write file (and nothing to stdout)
 ! stdout .
 cmpenv go.mod $WORK/go.mod.edit6
 
+# go mod edit -module
+cd $WORK/m
+go mod init a.a/b/c
+go mod edit -module x.x/y/z
+cmpenv go.mod go.mod.edit
+
 -- x.go --
 package x
 
@@ -159,3 +165,7 @@ exclude x.1     v1.2.0
 replace x.1    =>   y.1/v2 v2.3.6
 
 require x.3   v1.99.0
+-- $WORK/m/go.mod.edit --
+module x.x/y/z
+
+go $goversion
\ No newline at end of file