Simplify check_files

[ranjitjhala]

We should represent the configuration field check_files as

    check_files: Paths,

where

#[derive(Default)]
struct Paths {
    paths: Vec<PathBuf>,
}

impl<'de> Deserialize<'de> for Paths {
    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
    where
        D: serde::Deserializer<'de>,
    {
        let paths = <&str>::deserialize(deserializer)?
            .split(',')
            .into_iter()
            .map(PathBuf::from)
            .collect();
        Ok(Paths { paths })
    }
}

Originally posted by @nilehmann in #917 (comment)