Replace RE2 import with a dependency

third_party/re2/doc/mksyntaxgo

-#!/bin/sh
-
-set -e
-out=$GOROOT/src/regexp/syntax/doc.go
-cp syntax.txt $out
-sam -d $out <<'!'
-,x g/NOT SUPPORTED/d
-/^Unicode character class/,$d
-,s/[«»]//g
-,x g/^Possessive repetitions:/d
-,x g/\\C/d
-,x g/Flag syntax/d
-,s/.=(true|false)/flag &/g
-,s/^Flags:/  Flag syntax is xyz (set) or -xyz (clear) or xy-z (set xy, clear z). The flags are:\n/
-,s/\n\n\n+/\n\n/g
-,x/(^.* .*\n)+/ | awk -F'       ' '{printf("  %-14s %s\n", $1, $2)}'
-1,2c
-// Copyright 2012 The Go Authors.  All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-// DO NOT EDIT. This file is generated by mksyntaxgo from the RE2 distribution.
-
-/*
-Package syntax parses regular expressions into parse trees and compiles
-parse trees into programs. Most clients of regular expressions will use the
-facilities of package regexp (such as Compile and Match) instead of this package.
-
-Syntax
-
-The regular expression syntax understood by this package when parsing with the Perl flag is as follows.
-Parts of the syntax can be disabled by passing alternate flags to Parse.
-
-.
-$a
-*/
-package syntax
-.
-w
-q
-!