Optimize crawl_build_deps to not re-traverse deps

Optimize crawl_build_deps to not re-traverse deps

This function previously, for each dependency, traversed the entire dependency
graph. This ended up taking quite a bit of time as a huge amount of hashing was
being done (each of which is somewhat nontrivial). This commit alters the logic
to instead precompute a table of all the native dependencies that each crate
will need, traversing the dependency graph only once instead of repeatedly.

This commit takes a noop build of Servo from 2.1s to 0.4s