Warn against potentially overly nested depsets

[laurentlb]

In Bazel, depsets are not always used efficiently. API changes (bazelbuild/bazel#5817) encourage users to write the efficient solution.

We can statically detect bad patterns. For example, this is sometimes used to add things to a depset:

x = depset(..., transitive = [..., x, ...])

If this pattern is used inside a for loop, we should report a warning because it can lead to overly nested depsets (which lead to performance problems).

[brandjon]

Nice! @c-parsons and I were talking about this idea just the other day, had no idea you were already implementing it!