resolve_source_as_path always copies to temp

As implemented in #79 the `resource_source_as_path` now always copies the source file into a (potentially temporary) workdir, even if the file is a local file and already has a path. This means that when one runs `docling *.pdf` with a lot of pdfs in the current working directory, docling spends a lot of time copying them over to a temporary directory. In one of my use cases the `/var/tmp` has per-user quotas and docling processing with a lot of PDFs has reached that quota and errored-out. I believe the default should be to not do a copy, or at least, there should be a way to request not doing a copy (via the CLI and also the library).