all: freeze and deprecate more packages

Fixes golang/go#65250

Change-Id: I6a6a6964a2c87e529be50dd67fec462483b07b75
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/701535
Reviewed-by: Mark Freeman <markfreeman@google.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

ssh/test/doc.go

@@ -4,4 +4,6 @@
 
 // Package test contains integration tests for the
 // golang.org/x/crypto/ssh package.
+//
+// Deprecated: this package is for internal use only.
 package test