read_packed_refs(): do more of the work of reading packed refs

read_packed_refs(): do more of the work of reading packed refs

Teach `read_packed_refs()` to also

* Allocate and initialize the new `packed_ref_cache`
* Open and close the `packed-refs` file
* Update the `validity` field of the new object

This decreases the coupling between `packed_refs_cache` and
`files_ref_store` by a little bit.

Signed-off-by: Michael Haggerty <redacted>
Signed-off-by: Junio C Hamano <redacted>