defer tshark check from import time to runtime

[Rameen-Mahmood]

Changes

• moved tshark detection from module-level into a _find_tshark() helper called at runtime
• previously, import pcap_parser would sys.exit() if tshark was not installed, making the package unusable as a library and breaking flow.py (which does not need tshark)
• run_tshark and extract_dhcp_hostnames now accept an optional tshark_path param, looked up lazily by default

Testing

• all 27 tests pass
• backwards compatible, existing callers do not need to change