PR #17258 extracted `derp.Server` into `derp/derpserver.Server`.
This followup patch adds the following cleanups:
1. Rename `derp_server*.go` files to `derpserver*.go` to match
the package name.
2. Rename the `derpserver.NewServer` constructor to `derpserver.New`
to reduce stuttering.
3. Remove the unnecessary `derpserver.Conn` type alias.
Updates #17257
Updates #cleanup
Signed-off-by: Simon Law <sfllaw@tailscale.com>
