Don't require pack-file existence in git_packfile_check

See https://github.com/libgit2/libgit2/issues/1737

I have a situation where I download index files, but not their
corresponding pack files, from Amazon S3 because I want to determine
which objects live in which pack files, so that I know which one to
download without having to download them all. However, if I call
git_odb_backend_one_pack with just an index file, it gives me an error
right away saying that the pack-file doesn't also exist.

I see no reason why the check for pack-file existence can't be deferred
until the time when its actually needs to be opened to get the object
data. This makes it possible for people to do things that only require
the index data with just the index.

Note that I've made this change locally, and it works great. I can now
catalog the contents of pack files without downloading pack files (only
the indexes), and then later download the pack file that I know contains
the object(s) I need.