Make gonet.PacketConn implement net.Conn.
Make gonet.PacketConn implement net.Conn.

gonet.PacketConn now implements net.Conn, allowing it to be returned from
net.Dial.Dialer functions.

PiperOrigin-RevId: 238111980
Change-Id: I174884385ff4d9b8e9918fac7bbb5b93ca366ba7

google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor