class Get

Gets the package directory if it is present on the local system. If it is not, the

`missing_blobs` iterator will provide all the blobs in the package that are missing from

the system, and the ability to write those blobs to blobfs. If all the missing blobs are

downloaded and written to by the client, the `dir` directory will be resolved. This method

will return successfully when the package has been fully resolved, or return an error if

the client closes `needed_blobs` or `dir` handle before the package has been resolved.

This method does not guarantee the missing blobs have been persisted. In order to guarantee

missing blobs are persisted, clients should call ['Sync'].

Clients must not make concurrent `Get()` calls for the same `meta_far_blob`, even across

different `PackageCache` connections, *unless* the `meta_far_blob` is in base or already

active in the dynamic index. Violating this may result in `Get()` errors.

+ request `meta_far_blob` the blob info for the package's meta.far.

+ request `needed_blobs` an iterator over all the blobs in the package that

are not present on the system.

+ request `dir` the channel on which the package directory will be served.

* error a zx_status value indicating failure. One of the following:

* `ZX_ERR_UNAVAILABLE` if the client closed `needed_blobs` handles before

all the missing blobs were downloaded to the system.