dump_breakpad_symbols.py's parallelism doesn't work with RPM/DEBs

When using the "r" or "-pkg" option, dump_breakpad_symbols.py is extracting the RPM/DEB into a temporary directory. The lifetime of that temporary directory is maintained by yielding tuples from enumerate_pkg_files(). When using parallelism (added in IMPALA-11511), the yield doesn't keep the temporary directory around while the parallel threads are processing, so they fail with:

Found debugging info in /tmp/tmpqfZ9MZ/usr/lib/debug/usr/lib/impala/sbin-retail/impalad.debug
Failed to open ELF file '/tmp/tmpqfZ9MZ/usr/lib/debug/usr/lib/impala/sbin-retail/impalad.debug': No such file or directory
Failed to write symbol file.

Testing shows that this is still a problem with num_processes=1, so there should also be a change to be able to turn off the ThreadPool entirely. Processing OS packages can force the parallelism off for now as they don't benefit much from parallelism.