namespaced-features panics on unknown feature name

[ehuss]

Problem
Cargo panics with an unknown feature name when specifying a feature that doesn't exist when namespaced-features is enabled.

Steps
Create a project with this:

cargo-features = ["namespaced-features"]

[package]
name = "foo"
version = "0.1.0"
namespaced-features = true

Run cargo build --features=abc.

thread 'main' panicked at 'must be a valid feature', src/tools/cargo/src/cargo/core/resolver/dep_cache.rs:442:19

backtrace

   9: cargo::core::resolver::dep_cache::Requirements::require_feature
  10: cargo::core::resolver::dep_cache::RegistryQueryer::build_deps
  11: cargo::core::resolver::activate
  12: cargo::core::resolver::activate_deps_loop
  13: cargo::core::resolver::resolve
  14: cargo::ops::resolve::resolve_with_previous
  15: cargo::ops::resolve::resolve_ws_with_opts
  16: cargo::ops::cargo_compile::compile_ws
  17: cargo::ops::cargo_compile::compile
  18: cargo::commands::build::exec
  19: cargo::cli::main
  20: cargo::main

Notes

cargo 1.44.0-nightly (7019b3ed3 2020-03-17)

cc @djc