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

.golangci.yml

@@ -10,6 +10,10 @@ issues:
       linters:
         - dupl
         - maligned
+    - path: internal/os/alias\.go
+      linters:
+        - gochecknoglobals
+      text: IsNotExist is a global variable
 
 linters:
   disable-all: true