cmd/vet: move cmd/vet/whitelist to cmd/vet/internal/whitelist

This was a mistake made when bringing cmd/vet into the main repo.

Fixes #13416.

Change-Id: I03b512ab944577c56085aea06df8ff5e1acc16d7
Reviewed-on: https://go-review.googlesource.com/17455Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/vet/composite.go

@@ -7,7 +7,7 @@
 package main
 
 import (
-	"cmd/vet/whitelist"
+	"cmd/vet/internal/whitelist"
 	"flag"
 	"go/ast"
 	"strings"

src/cmd/vet/whitelist/whitelist.go → src/cmd/vet/internal/whitelist/whitelist.go

@@ -3,7 +3,7 @@
 // license that can be found in the LICENSE file.
 
 // Package whitelist defines exceptions for the vet tool.
-package whitelist // import "cmd/vet/whitelist"
+package whitelist
 
 // UnkeyedLiteral are types that are actually slices, but
 // syntactically, we cannot tell whether the Typ in pkg.Typ{1, 2, 3}