client/tailscale/apitype: move local API types to new apitype package

They were scattered/duplicated in misc places before.

It can't be in the client package itself for circular dep reasons.

This new package is basically tailcfg but for localhost
communications, instead of to control.

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/depaware.txt

@@ -15,6 +15,7 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
         rsc.io/goversion/version                                     from tailscale.com/version
         tailscale.com/atomicfile                                     from tailscale.com/ipn
         tailscale.com/client/tailscale                               from tailscale.com/cmd/tailscale/cli
+        tailscale.com/client/tailscale/apitype                       from tailscale.com/client/tailscale
         tailscale.com/cmd/tailscale/cli                              from tailscale.com/cmd/tailscale
         tailscale.com/derp                                           from tailscale.com/derp/derphttp
         tailscale.com/derp/derphttp                                  from tailscale.com/net/netcheck