Complete IRC numerics module and move to pkg/irc/ (refs #52)

- Add all missing numeric reply codes from RFC 1459 and RFC 2812
- Move irc package from internal/irc/ to pkg/irc/ for external use
- Update all imports throughout the codebase
- Added: trace replies (200-209), stats replies (211-219, 242-243),
  admin replies (256-259), service replies (234-235), WHOWAS (314, 369),
  list start (321), version (351), links (364-365), info (371, 374),
  oper/rehash/service (381-383), time/users (391-395),
  invite/except lists (346-349), unique ops (325),
  and all missing error codes (406-415, 422-424, 436-437, 443-446,
  463-467, 472, 476-478, 481, 483-485, 491, 501-502)

pkg/irc/commands.go

@@ -0,0 +1,21 @@
+package irc
+
+// IRC command names (RFC 1459 / RFC 2812).
+const (
+	CmdJoin    = "JOIN"
+	CmdList    = "LIST"
+	CmdLusers  = "LUSERS"
+	CmdMode    = "MODE"
+	CmdMotd    = "MOTD"
+	CmdNames   = "NAMES"
+	CmdNick    = "NICK"
+	CmdNotice  = "NOTICE"
+	CmdPart    = "PART"
+	CmdPing    = "PING"
+	CmdPong    = "PONG"
+	CmdPrivmsg = "PRIVMSG"
+	CmdQuit    = "QUIT"
+	CmdTopic   = "TOPIC"
+	CmdWho     = "WHO"
+	CmdWhois   = "WHOIS"
+)