Panic in `cargo doc` with -Zfeatures=itarget

[ehuss]

Not sure if this was mentioned and/or expected but it seems that doc command is not adapted yet. For the manifest:

[package]
name = "sequoia-openpgp-default"
version = "0.1.0"
authors = ["Sequoia Developers <dev@sequoia-pgp.org>"]
edition = "2018"

[target.'cfg(not(windows))'.dependencies.sequoia-openpgp]
git = "https://gitlab.com/sequoia-pgp/sequoia"
default-features = false
features = ["crypto-nettle"]

[target.'cfg(windows)'.dependencies.sequoia-openpgp]
git = "https://gitlab.com/sequoia-pgp/sequoia"
default-features = false
features = ["crypto-cng"]

I got

PS C:\Users\Xanewok\repos\sequoia\openpgp-default> rustc +nightly --version --verbose
rustc 1.49.0-nightly (8dae8cdcc 2020-10-12)
binary: rustc
commit-hash: 8dae8cdcc8fa879cea6a4bbbfa5b32e97be4c306
commit-date: 2020-10-12
host: x86_64-pc-windows-gnu
release: 1.49.0-nightly
LLVM version: 11.0
PS C:\Users\Xanewok\repos\sequoia\openpgp-default> cargo +nightly --version
cargo 1.48.0-nightly (9d1a4863a 2020-10-05)
PS C:\Users\Xanewok\repos\sequoia\openpgp-default> cargo +nightly check --no-default-features -Zfeatures=itarget
    Finished dev [unoptimized + debuginfo] target(s) in 0.17s
PS C:\Users\Xanewok\repos\sequoia\openpgp-default> cargo +nightly doc --no-default-features -Zfeatures=itarget  
thread 'main' panicked at 'activated_features for invalid package: features did not find PackageId { name: "nettle", version: "7.0.0", source: "registry `https://github.com/rust-lang/crates.io-index`" } false', src\tools\cargo\src/cargo\core\resolver\features.rs:227:14
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

which at a first glance seems that activated_features contains an entry for the nettle package, which should only be activated under cfg(not(windows)) and is correctly skipped for the check build.

Originally posted by @Xanewok in https://github.com/rust-lang/cargo/issue_comments/707717140