Omit any file that is not a zip when repackaging

When repackaging an archive, the files in the resulting lib directory
must be zip files. If they're not zip files, the resulting archive
may fail to run (#324).

The previous approach was to consider an artifact's type when deciding
whether or not it should be packaged. The type is a string and, while
there are a number of well-known values, it can essentially be anything.
This caused a problem with an artifact incorrectly being identified as
being unsuitable for inclusion (#489).

This commit changes the approach. Rather than looking at an artifact's
type, it looks at the first four bytes  of the archive's file. Only if
these header bytes matche that of a zip file is the artifact included.
This is a better match for the requirement that all files in lib be zip
files.

Fixes #489