loose: refactor object map to operate on `struct odb_source_loose`

loose: refactor object map to operate on `struct odb_source_loose`

While the loose object map functions in "loose.c" accept a generic
`struct odb_source *`, they always expect this to be the "files"
backend. Furthermore, the subsystem doesn't even care about the "files"
backend, but only uses it as a stepping stone to get to the "loose"
backend.

This assumption is implicit and thus not immediately obvious. Refactor
the interfaces to instead operate on a `struct odb_source_loose`
instead, which eliminates the implicit dependency and unnecessary detour
via the "files" source.

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