cmd/go: support workspaces in vet

Add modload.InitWorkfile to runVet so that the vet command recognizes
and uses the workspace.

Fixes #51072

Change-Id: Ia6727eff9b80eb33627f5ae23e4d72cde581e75f
Reviewed-on: https://go-review.googlesource.com/c/go/+/385176
Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/go/internal/vet/vet.go

@@ -13,6 +13,7 @@ import (
 	"cmd/go/internal/base"
 	"cmd/go/internal/cfg"
 	"cmd/go/internal/load"
+	"cmd/go/internal/modload"
 	"cmd/go/internal/trace"
 	"cmd/go/internal/work"
 )
@@ -54,6 +55,7 @@ See also: go fmt, go fix.
 
 func runVet(ctx context.Context, cmd *base.Command, args []string) {
 	vetFlags, pkgArgs := vetFlags(args)
+	modload.InitWorkfile() // The vet command does custom flag processing; initialize workspaces after that.
 
 	if cfg.DebugTrace != "" {
 		var close func() error

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

@@ -0,0 +1,19 @@
+! go vet ./a
+stderr 'fmt.Println call has possible formatting directive'
+
+-- go.work --
+go 1.18
+
+use ./a
+-- a/go.mod --
+module example.com/a
+
+go 1.18
+-- a/a.go --
+package a
+
+import "fmt"
+
+func A() {
+    fmt.Println("%s")
+}