Make dep_variant_infos a list, not a depset

[dzbarsky]

For anyone using rust_library_group (or similar, like rules_rs) the current setup forces flattening in every rdep. We can just carry them as a list instead and iterate it directly in the consumers.

We are slightly less efficient now when nesting a library group in a library group (which is probably very rare?).

The prost rules do incur flattening now when include_transitive_deps is enabled (which it's not by default, and it seems like a bad practice to not declare your deps properly?). In exchange, I've improved the depset handling for constructing those transitive deps (and I think we could further improve it by not constructing the transitive depset when the toolchain isn't configured to use it...)

This is technically a breaking change for folks who have custom rules creating CrateGroupInfo, although the adjustment should be pretty trivial.