debuginfod: suppress fdcache prefetching during dwz lookup

debuginfod: suppress fdcache prefetching during dwz lookup

During a recent from-scratch reindexing of the rpm/deb corpus at
debuginfod.elfutils.org, we found the fdcache chewed up an abnormal
amount of $TMPDIR space.  This was due to internal .dwz lookups, which
triggered fdcache prefetching as for a webapi query, but there was not
a timely fdcache eviction pass to clean it up again.  Rather than add
that pass, it's better to suppress the prefetching completely, as an
internal .dwz search will only ever need that file, not any others
from the same archive.

Signed-off-by: Frank Ch. Eigler <fche@redhat.com>