This makes tsnet apps not depend on x/crypto/ssh and locks that in with a test.
It also paves the wave for tsnet apps to opt-in to SSH support via a
blank feature import in the future.
Updates #12614
Change-Id: Ica85628f89c8f015413b074f5001b82b27c953a9
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>