refactor(config): rename package _config to config with cfg alias

The config package used `package _config` with a leading underscore,
which is unconventional in Go. Rename to `package config` (matching the
directory name) and use `cfg` as the standard import alias across all
93 importing files.

network/clientctx/clientcontext.go

@@ -1,8 +1,8 @@
 package clientctx
 
-import "erupe-ce/config"
+import cfg "erupe-ce/config"
 
 // ClientContext holds contextual data required for packet encoding/decoding.
 type ClientContext struct {
-	RealClientMode _config.Mode
+	RealClientMode cfg.Mode
 }