packfile: use a `strmap` to store packs by name

packfile: use a `strmap` to store packs by name

To allow fast lookups of a packfile by name we use a hashmap that has
the packfile name as key and the pack itself as value. But while this is
the perfect use case for a `strmap`, we instead use `struct hashmap` and
store the hashmap entry in the packfile itself.

Simplify the code by using a `strmap` instead.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>