Improve resilience to broken zkg.meta when aggregating a package source

[ckreibich]

It'd be good to handle this more gracefully: https://github.com/zeek/packages/actions/runs/3274626082/jobs/5388405156

Copied for reference:

2022-10-18 15:07:00 DEBUG    init Manager version 2.13.0
2022-10-18 15:07:00 DEBUG    creating source clone of "zeek" at /home/runner/.zkg/clones/source/zeek
2022-10-18 15:07:00 DEBUG    refresh "zeek": pulling ***github.com/zeek/packages
2022-10-18 15:07:01 DEBUG    aggegrate file in source unchanged, restore local one
Refresh package source: zeek
Traceback (most recent call last):
  File "/home/runner/.local/bin/zkg", line 2770, in <module>
    main()
  File "/home/runner/.local/bin/zkg", line 2767, in main
    args.run_cmd(manager, args, config, configfile)
  File "/home/runner/.local/bin/zkg", line 1189, in cmd_refresh
    res = manager.aggregate_source(source, args.push)
  File "/home/runner/.local/lib/python3.8/site-packages/zeekpkg/manager.py", line 865, in aggregate_source
    return self._refresh_source(name, True, push)
  File "/home/runner/.local/lib/python3.8/site-packages/zeekpkg/manager.py", line 1006, in _refresh_source
    invalid_reason = _parse_package_metadata(
  File "/home/runner/.local/lib/python3.8/site-packages/zeekpkg/manager.py", line 2992, in _parse_package_metadata
    if not parser.read(metadata_file):
  File "/usr/lib/python3.8/configparser.py", line 697, in read
    self._read(fp, filename)
  File "/usr/lib/python3.8/configparser.py", line 1082, in _read
    raise MissingSectionHeaderError(fpname, lineno, line)
configparser.MissingSectionHeaderError: File contains no section headers.
file: '/home/runner/.zkg/scratch/zeek-outbound-known-services-with-origflag/zkg.meta', line: 1
'package]\n'

It'd also be good to do basic package sanity checking at the zkg level when adding a package to the default source.