net/http: replace SOCKS client implementation

This change replaces the vendored socks client implementation with the
bundle of golang.org/x/net/internal/socks package which contains fixes
for 19354 and 11682.

golang.org/x/net/internal/socks becomes socks_bundle.go.

At git rev 61147c4. (golang.org/cl/38278)

Updates #11682.
Updates #18508.
Updates #19354.
Fixes #19688.
Updates #21333.

Change-Id: I8cf6c3f5eb87c24685a7592be015729f84fbed77
Reviewed-on: https://go-review.googlesource.com/41031
Run-TryBot: Mikio Hara <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: cixtor

src/go/build/deps_test.go

@@ -403,7 +403,6 @@ var pkgDeps = map[string][]string{
 		"golang_org/x/net/http2/hpack",
 		"golang_org/x/net/idna",
 		"golang_org/x/net/lex/httplex",
-		"golang_org/x/net/proxy",
 		"golang_org/x/text/unicode/norm",
 		"golang_org/x/text/width",
 		"internal/nettrace",