odb: write alternates via sources

odb: write alternates via sources

Refactor writing of alternates so that the actual business logic is
structured around the object database source we want to write the
alternate to. Same as with the preceding commit, this will eventually
allow us to have different logic for writing alternates depending on the
backend used.

Note that after the refactoring we start to call
`odb_add_alternate_recursively()` unconditionally. This is fine though
as we know to skip adding sources that are tracked already.

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