Fix for inactive, optional dependencies in guppy 0.13

[sunshowers]

This issue tracks the Cargo build simulation fix for inactive, optional dependencies in guppy 0.13 (commit e03f119).

More about this:

[package]
resolver = "2"
name = "main"

[target.'cfg(unix)'.dependencies]
dep = { version = "...", optional = true }

[features]
feat = ["dep/feat"]

Previously, the feature main/feat -> main/dep was activated unconditionally. This did not match Cargo's behavior, which only activated main/feat -> main/dep if dep was already active.

With guppy 0.13, guppy now matches Cargo's behavior.

This also causes a minor behavior change to hakari (#586).