cmd/deadcode: publish internal/cmd/deadcode

Fixes golang/go#63501

Change-Id: I794734e4508d7ccbd8653554f14d109ee8ca9927
Reviewed-on: https://go-review.googlesource.com/c/tools/+/547035
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>

internal/cmd/deadcode/doc.go → cmd/deadcode/doc.go

@@ -100,8 +100,8 @@ the formatting of the list of Edge objects.
 The default format shows, for each edge in the path, whether the call
 is static or dynamic, and its source line number. For example:
 
-	$ deadcode -whylive=bytes.Buffer.String -test ./internal/cmd/deadcode/...
-	                 golang.org/x/tools/internal/cmd/deadcode.main
+	$ deadcode -whylive=bytes.Buffer.String -test ./cmd/deadcode/...
+	                 golang.org/x/tools/cmd/deadcode.main
 	static@L0117 --> golang.org/x/tools/go/packages.Load
 	static@L0262 --> golang.org/x/tools/go/packages.defaultDriver
 	static@L0305 --> golang.org/x/tools/go/packages.goListDriver