go/analysis: allow overriding V flag without code patches

In CL 149609, a file was added to
src/cmd/vendor/golang.org/x/tools/go/analysis/internal/analysisflags/patch.go
to override the behavior of the V flag for cmd/vet.

That modification causes the behavior of cmd/vet to change when a
pristine copy of x/tools is vendored in, and module-mode vendoring
will only support pristine copies (see golang/go#30240).

Instead, allow cmd/vet to override the V flag by defining its own V
flag before it invokes unitchecker.Main.

Tested manually (by patching into cmd/vendor).

Updates golang/go#30240
Updates golang/go#30241
Updates golang/go#26924
Updates golang/go#30228

Change-Id: I10e4523e1f4ede94fbfc745012dadeefef48e927
Reviewed-on: https://go-review.googlesource.com/c/162989
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

go/analysis/internal/analysisflags/flags.go

@@ -152,12 +152,13 @@ func printFlags() {
 // addVersionFlag registers a -V flag that, if set,
 // prints the executable version and exits 0.
 //
-// It is a variable not a function to permit easy
-// overriding in the copy vendored in $GOROOT/src/cmd/vet:
-//
-// func init() { addVersionFlag = objabi.AddVersionFlag }
-var addVersionFlag = func() {
-	flag.Var(versionFlag{}, "V", "print version and exit")
+// If the -V flag already exists — for example, because it was already
+// registered by a call to cmd/internal/objabi.AddVersionFlag — then
+// addVersionFlag does nothing.
+func addVersionFlag() {
+	if flag.Lookup("V") == nil {
+		flag.Var(versionFlag{}, "V", "print version and exit")
+	}
 }
 
 // versionFlag minimally complies with the -V protocol required by "go vet".