cmd/go: fix go mod tidy crash on empty module

Fixes #27066.

Change-Id: Iede4385ad86b42d7d90814965b161a7e64d29833
Reviewed-on: https://go-review.googlesource.com/129799
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/internal/modcmd/tidy.go

@@ -44,10 +44,11 @@ func runTidy(cmd *base.Command, args []string) {
 
 	// LoadALL adds missing modules.
 	// Remove unused modules.
-	used := map[module.Version]bool{modload.Target: true}
+	used := make(map[module.Version]bool)
 	for _, pkg := range modload.LoadALL() {
 		used[modload.PackageModule(pkg)] = true
 	}
+	used[modload.Target] = true // note: LoadALL initializes Target
 
 	inGoMod := make(map[string]bool)
 	for _, r := range modload.ModFile().Require {

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

@@ -10,6 +10,10 @@ go list -m all
 stdout '^w.1 v1.2.0'
 stdout '^z.1 v1.2.0'
 
+# empty tidy should not crash
+cd triv
+go mod tidy
+
 -- go.mod --
 module m
 
@@ -55,3 +59,6 @@ module z
 
 -- z/sub/sub.go --
 package sub
+
+-- triv/go.mod --
+module triv