Fix: unmarshal key values in Java, Go, and Conan metadata (#2603)

Previously, Syft represented several metadata fields as map[string]string,
however this representation erased ordering, so Syft now represents these values
as []KeyValue. Add custom unmarshaling so that JSON that was written by
older versions of Syft using the map[string]string representation can be parsed
into the new []KeyValue representation.

Signed-off-by: Will Murphy <will.murphy@anchore.com>

syft/pkg/key_value.go

@@ -1,5 +1,11 @@
 package pkg
 
+import (
+	"encoding/json"
+	"fmt"
+	"sort"
+)
+
 type KeyValue struct {
 	Key   string `json:"key"`
 	Value string `json:"value"`
@@ -26,3 +32,39 @@ func (k KeyValues) MustGet(key string) string {
 
 	return ""
 }
+
+func keyValuesFromMap(m map[string]string) KeyValues {
+	var result KeyValues
+	var mapKeys []string
+	for k := range m {
+		mapKeys = append(mapKeys, k)
+	}
+	sort.Strings(mapKeys)
+	for _, k := range mapKeys {
+		result = append(result, KeyValue{
+			Key:   k,
+			Value: m[k],
+		})
+	}
+	return result
+}
+
+func (k *KeyValues) UnmarshalJSON(b []byte) error {
+	var kvs []KeyValue
+	if err := json.Unmarshal(b, &kvs); err != nil {
+		var legacyMap map[string]string
+		if err := json.Unmarshal(b, &legacyMap); err != nil {
+			return fmt.Errorf("unable to unmarshal KeyValues: %w", err)
+		}
+		var keys []string
+		for k := range legacyMap {
+			keys = append(keys, k)
+		}
+		sort.Strings(keys)
+		for _, k := range keys {
+			kvs = append(kvs, KeyValue{Key: k, Value: legacyMap[k]})
+		}
+	}
+	*k = kvs
+	return nil
+}