bup-server: revert to non-midx indexes when suggesting a pack.

bup-server: revert to non-midx indexes when suggesting a pack.

Currently midx files can't tell you *which* index contains a particular
hash, just that *one* of them does.  So bup-server was barfing when it
expected MultiPackIndex.exists() to return a pack name, and was getting a
.midx file instead.

We could have loosened the assertion and allowed the server to suggest a
.midx file... but those can be huge, and it defeats the purpose of only
suggesting the minimal set of packs so that lightweight clients aren't
overwhelmed.