Remove MetadataType from the core package struct

[wagoodman]

(reporting on the behalf of @kzantow) The only reason to have a MetadataType field is so that when marshalled to JSON, there is an identifier available to determine which type to unmarshal to. However, this can be entirely handled in the JSON encoding/decoding using something similar to:

func (p Package) MarshalJSON() ([]byte, error) {
    type pk Package
    return json.Marshal(struct {
        pk
        MetadataType string
    }{
        pk: pk(p),
        MetadataType:    reflect.TypeOf(p.Metadata).Name(),
    })
}

func (b *Package) UnmarshalJSON(data []byte) error {
    var typ struct {
        MetadataType string
    }
    if err := json.Unmarshal(data, &typ); err != nil {
        return err
    }

    metadataType := pkg.MetadataTypeByName[typ.MetadataType]

    type tmp Package // avoids infinite recursion
    err := json.Unmarshal(data, (*tmp)(p))
    if err != nil { return err }

    // handle metadata

    return nil
}

[wagoodman]

Should be done in conjunction with #1844 . Also consider consolidating all instances where a user would need to make an addition when a new metadata struct is added to also be generated by the same method as done in #1841 .