`cargo metadata` doesn't indicate that feature-enabled transitive platform-specific deps are platform-specific

[illicitonion]

Problem
If on one platform a crate enables a feature which enables a transitive dependency, and on another it doesn't, there's no way to capture this from the output of cargo metadata.

Steps

1. In this repo run cargo metadata or cargo metadata --filter-platform=x86_64-unknown-linux-gnu. In this crate, serde's derive feature is enabled for x86_64-apple-darwin but not for x86_64-unknown-linux-gnu, which means it depends on serde-derive on x86_64-apple-darwin but not on x86_64-unknown-linux-gnu.
2. Try to work out whether serde-derive should be built on x86_64-unknown-linux-gnu.
3. Find that there's not enough information to determine this.

Possible Solution(s)

1. We could populate DepKindInfo.target for these dependencies, such that they can be identified as platform-specific
2. When running with --filter-platform for a non-matching platform, we could omit these dependencies. That may require callers to run cargo metadata multiple times and splice the results, which would be a bit unfortunate.

Notes

Output of cargo version: cargo 1.54.0 (5ae8d74b3 2021-06-22), also cargo 1.56.0-nightly (f559c109c 2021-08-26)

This issue makes it hard to translate crates.io packages for building with non-cargo build systems.