Don't load kexts/kernels without dSYMs present

One of the backup schemes I use for finding kexts and kernels
on the local filesystem is to load a solitary binary when I don't
find any with a dSYM.  This usually is a more confusing behavior
than helpful; people expect to get no binary loaded, or a binary
with debug information.  This change stops loading kexts and
kernels that do not have an associated dSYM.

rdar://74291888