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.
The .get() function raises a queue.Empty exception anyway, so there was
no real need for the .empty() call. Furthermore, it turned out to really
slow things down.
For this to work well I also had to remove the sorting of blocks in
trace.deps(). The sorting caused the first `yield` to be executed only
after each blend file was opened, which means that the consuming for-loop
takes a long time to hit its first iteration. As a result, it would respond
slowly to abort requests. By not sorting the first `yield` is much sooner,
resolving this issue.
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.
