change wgtunnel to import github.com/tailscale/wireguard-go

[spikecurtis]

tailscale has forked wireguard into a renamed module instead of the original golang.zx2c4.com/wireguard. We should update wgtunnel to use it and the new name since tailscale seem to be much more active maintainers.

c.f. https://github.com/tailscale/wireguard-go/tree/tailscale

[spikecurtis]

c.f. https://github.com/coder/wgtunnel/compare/spike/tailscale-wireguard

[spikecurtis]

fixed in #28