The target path is just read as string from the CLI now, to allow more
complex targets (such as URLs) that don't directly map to a path.
The Packer subclass now handles the conversion from that string to a
`pathlib.PurePath`, and specific subclasses & transfer classes can convert
those to a `pathlib.Path` to perform actual filesystem operations when
necessary.
For regular file transfers (so to a directory, not to a ZIP file or S3
storage), use multi-threaded transfer when compressing. Compressing is
CPU-bound, so using multiple threads speeds things up considerably
(packing a Spring lighting file went from 6min30 single-threaded to
2min13 multi-threaded on my machine).
Both the dependency Tracer class and the Packer class now support a
callback object, where the latter is a subclass of the former.
For file transfers running in a separate thread, there is a thread-safe
wrapper for progress callbacks. This wrapper can be called from any thread,
and calls the wrapped callback object from the main thread. This way the
callback implementation itself doesn't have to worry about threading
issues.
