refactor: use a map instead of a slice

Packages are unique anyways so it makes sense to use a map to avoid having
duplicates.

For compatibility reasons custom json (un)marshallers hide this change from the
end user

spec/spec.go

@@ -14,10 +14,42 @@
 
 package spec
 
+import (
+	"encoding/json"
+	"sort"
+)
+
 type JsonnetFile struct {
+	Dependencies map[string]Dependency
+}
+
+type jsonFile struct {
 	Dependencies []Dependency `json:"dependencies"`
 }
 
+func (jf *JsonnetFile) UnmarshalJSON(data []byte) error {
+	var s jsonFile
+	if err := json.Unmarshal(data, &s); err != nil {
+		return err
+	}
+
+	jf.Dependencies = make(map[string]Dependency)
+	for _, d := range s.Dependencies {
+		jf.Dependencies[d.Name] = d
+	}
+	return nil
+}
+
+func (jf JsonnetFile) MarshalJSON() ([]byte, error) {
+	var s jsonFile
+	for _, d := range jf.Dependencies {
+		s.Dependencies = append(s.Dependencies, d)
+	}
+
+
+	return json.Marshal(s)
+}
+
 type Dependency struct {
 	Name      string `json:"name"`
 	Source    Source `json:"source"`