To ensure that it just returns a single package record for some given
arguments, as this helps to avoid poor performance of the store connection
object cache.
Performance with computing the cross derivations is important for the data
service, as it's currently very slow to process revisions.
* gnu/packages/rust.scm (make-rust-sysroot): Move code to
make-rust-sysroot/implementation.
(make-rust-sysroot/implementation): New variable.
Change-Id: Ibb30c7398328c87c032bb8828635a34ada935167
d162bd0e72