Billy: Path traversal vulnerabilities

Billy is an interface filesystem abstraction for Go. Prior to 5.9.0, multiple path traversal issues exist across different components of go-billy. Insufficient path sanitization and boundary enforcement may allow crafted paths (e.g., using ..) to escape intended base directories. While go-billy was not originally designed to provide a strong security boundary, some of these issues were inconsistent across some of the built-in implementations. This results in scenarios where applications relying on go-billy for some level of isolation may inadvertently expose access to unintended filesystem locations. This vulnerability is fixed in 5.9.0.