Filesystem abstraction to handle all folder/file I/O.
Path to the local package cache (usually the one in the project), whih takes preference over user and system.
Path to the user package cache (usually ~/.dub/packages/), takes precedence over system but not over local.
Path to the system package cache, this has the least precedence.
Instantiate a PackageManager with the provided Filesystem and paths
Unlike the other overload, paths are taken as-if, e.g. packages/ is not appended to them.