buildsys: validate manifest metadata when deserializing

[webern]

The ManifestInfo type deserializes certain sections from Cargo.toml

• package.metadata.build-package
• package.metadata.build-kit
• package.metadata.build-variant

Currently the differences between build-package, build-kit and build-variant is not constrained by an enum. This leads to a lot of "late binding" errors if an expected field is missing. Instead a different type for each of these should be created. For example:

struct BuildPackageInfo { }
struct BuildKitInfo { }
struct BuildVariantInfo { }

enum BuildsysMetadata {
    Package(BuildPackageInfo),
    Kit(BuildKitInfo),
    Variant(BuildVariantInfo),
}

If serde is used correctly with this structure, then missing information will cause an error at deserialization time and get rid of Options and errors downstream when fields are being accessed.

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          We should create an issue, if we don't have one already, for deserializing these manifest metadata sections with Serde in a validating manner. We should catch these errors earlier.

Originally posted by @webern in #271 (comment)