Fix parse_cargo_tree_output misclassifying dev-dependencies as host deps (#3976)

Fix `parse_cargo_tree_output` incorrectly classifying
`[dev-dependencies]` as host dependencies when they follow
`[build-dependencies]` for the same crate.

This causes platform-conditional features to leak across platforms in
the generated BUILD files. For instance, the `tokio` feature `taskdump`
that can only be enabled on linux could leak to other platforms.

Example dependency tree that triggers the issue:
```
;my-crate v0.1.0 (/my-crate);;
`-- ;tokio v1.49.0;bytes,full,taskdump;
[build-dependencies]
`-- ;anyhow v1.0.100;default,std;
[dev-dependencies]
`-- ;tokio-test v0.4.4;;
    `-- ;tokio v1.49.0;bytes,full,taskdump; (*)
```

This PR also updates surrounding code in `parse_cargo_tree_output`. Let
me know if you'd prefer a minimal fix instead.