Code maintenance: OS package for file system

- OS custom internal package for file system interaction
- Remove fileManager external dependency
- Closer API to Go's native API on the OS
- Create directories at startup
- Better testability
- Move Unsetenv to os interface

internal/constants/permissions.go

@@ -1,8 +0,0 @@
-package constants
-
-import "os"
-
-const (
-	UserReadPermission      os.FileMode = 0400
-	AllReadWritePermissions os.FileMode = 0666
-)