FR(crate_universe): honor .bazelignore and REPO.bazel ignore_directories

[alexeagle]

The splicer can ignore crates (Cargo.toml files) that appear in certain fixed paths:

rules_rust/crate_universe/src/splicing/splicer.rs

Line 42 in 3585384

const IGNORE_LIST: &[&str] = &[".git", "bazel-*", ".svn"];

If a directory is bazel-ignored, then the splicer should skip that too.