Features declared in Cargo.toml can cause files to not get checked without clean

[davidlattimore]

If a crate has a non-empty [features] section, then it seems that not all targets get checked for matches unless the user runs "cargo clean" first.

[davidlattimore]

This appears to be an issue with cargo (or plausibly a misunderstanding of cargo on my part). I've raised rust-lang/cargo#5375. I could work around it by making rerast do a full cargo clean before it does a cargo check, but that feels very heavy handed and might slow down rerast quite a bit.