crypto/tls: move defaults into defaults.go

crypto/tls: move defaults into defaults.go

Fixes #65265
Updates #60790

Change-Id: Iaa5f475d614d3ed87f091c93a3f888b7eb3433f2
Reviewed-on: https://go-review.googlesource.com/c/go/+/587296
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Derek Parker <parkerderek86@gmail.com>