Maint: refactor internal/routing

- Split Go files better
- Reduce public API for exported errors

internal/routing/errors.go

@@ -0,0 +1,11 @@
+package routing
+
+import (
+	"errors"
+)
+
+var (
+	ErrLinkByIndex         = errors.New("cannot obtain link by index")
+	ErrLinkDefaultNotFound = errors.New("default link not found")
+	ErrRoutesList          = errors.New("cannot list routes")
+)