Deny unknown fields in package configurations

This helps catching errors in package configuration files (`pkg.toml`)
by rejecting unknown fields when loading the packages repository.
The `config` crate is a bit limited in this regard but luckily this can
be caught during deserialization via Serde [0][1].

The error output contains the path of the problematic package
configuration file, the name of the unknown field, and a list of the
valid/expected fields.

Fixes science-computing#188.

[0]: rust-cli/config-rs#450
[1]: https://serde.rs/container-attrs.html#deny_unknown_fields

Signed-off-by: Michael Weiss <michael.weiss@atos.net>

src/package/package.rs

@@ -24,6 +24,7 @@ use crate::util::docker::ImageName;
 use crate::util::EnvironmentVariableName;
 
 #[derive(Clone, Serialize, Deserialize, Getters)]
+#[serde(deny_unknown_fields)]
 pub struct Package {
     #[getset(get = "pub")]
     name: PackageName,