make data runtime-only to align with ecosystem norms

[ajwerner]

In the current rules_rust rules, the data attribute is available both at compile time and at runtime whereas the compile_data is just available at runtime. There is no attribute that describes runtime-only dependencies. This is unfortunate; changing runtime dependencies shouldn't force a recompilation and relinking of an executable.

In order to align with the rest of the bazel ecosystem, we should change data to not be available at compile time. This will be a breaking change, but it's worth it.

[illicitonion]

I think this makes sense to do - it was a mistake that we used data for both back in the past.

Ideally we should introduce a flag to control this behaviour to make migration easier. Introduce the flag (I don't really mind what default we use), and delete the flag a release or two later when people have had an opportunity to migrate.

Happy to review a PR doing this!

[dzbarsky]

I can send a PR but I may not get to it for a few weeks. If anyone else is motivated in the meantime, go for it