go gen improvements (#524)

Remove the "gen" directory and move directives_generate.go out of it.
Add a build ignore tag so it isn't build by default. Cleanup the go gen
invocations so there are not seen as package docs.

Simplify the code a bit and don't run go gen twice.

core/dnsserver/directives.go

@@ -1,4 +1,3 @@
-//go:generate go run ../../gen/directives_generate.go ../../middleware.cfg
 package dnsserver
 
 import (