refactor: extract the state module and make the backoff error instead of panic

This PR:
- moves state.go to its own module
- moves the phpheaders test the phpheaders module
- simplifies backoff.go
- makes the backoff error instead of panic (so it can be tested)
- removes some unused C structs

internal/phpheaders/phpheaders.go

@@ -1,5 +1,6 @@
 package phpheaders
 
+import "C"
 import (
 	"context"
 	"strings"