Enable unpack/repack of .pkg containers

[mmitche]

.pkg files contain binaries that need to be signed. Today we unpack and repack these in staging, so we can sign and notarize the executables within. Expand support for this in signtool.

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.