Adds a check for missing installer items in pkginfo files

[AaronBurchfield]

This new check will detect pkginfo files that specify installer items that do not exist. We also identify installer items with file paths that are not the same case as the item on disk, since most Linux filesystems and web servers are case-sensitive by default and will fail to serve these files.

[homebysix]

@AaronBurchfield - I'm getting this traceback on my Munki repo with this hook change. My repo is not on a case-sensitive filesystem.

Traceback (most recent call last):
  File "/var/folders/0n/svv6yz7d5qlf59695s39bnx40000gn/T/tmpficw_tv8/repotfuceba2/py_env-python3.12/bin/check-munki-pkgsinfo", line 8, in <module>
    sys.exit(main())
             ^^^^^^
  File "/private/var/folders/0n/svv6yz7d5qlf59695s39bnx40000gn/T/tmpficw_tv8/repotfuceba2/py_env-python3.12/lib/python3.12/site-packages/pre_commit_hooks/check_munki_pkgsinfo.py", line 139, in main
    os.path.join("pkgs", pkginfo.get("installer_item_location"))
  File "<frozen posixpath>", line 90, in join
  File "<frozen genericpath>", line 164, in _check_arg_types
TypeError: join() argument must be str, bytes, or os.PathLike object, not 'NoneType'

[homebysix]

Addressed in bb51cf7, which will be part of 1.14.1.