Use depset instead of mutable lists

[hlopko]

Because of the implementation of rust_test we have to provide srcs, deps, and proc_macro_deps in CrateInfo so the test can fetch those and pass them together with its own sources and dependencies to rustc.

Currently a Rust library provides 2 providers: (1) CrateInfo describing the target, and (2) DepInfo describing its dependencies. The first thing that downstream libraries do is they get transitive depset from DepInfo, and put CrateInfo as direct.

This could be improved by putting CrateInfo into DepInfo already in the target that provides it - there would be only one depset created, not one per downstream dependency. The problem is though that for rust_test we cannot put the CrateInfo into the depset because Starlark sees it as mutable, and the cause is the use of a concatenated list. How come it worked before? Well, before the list came from a provider from a different target, so it was frozen. Now we are creating the provider, and our lists are not frozen.

I tried to lament this a bit and IMHO using depsets (which are frozen upon construction) is the best immediate step. I have a followup PR that modifies DepInfo so linking with complicated C++ dependency graphs works correctly, and that's where I encountered this problem (and that's where I extracted this PR from).