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/os/alias.go

@@ -0,0 +1,9 @@
+package os
+
+import nativeos "os"
+
+// Aliases used for convenience so "os" does not have to be imported
+
+type FileMode nativeos.FileMode
+
+var IsNotExist = nativeos.IsNotExist