Add package buildtimestamp and luet upgrade --sync

Annotate the package build time when compiling, and use that from the
client to force upgrade of packages that changed the artifact, but
didn't changed any version.

The client can trigger this behavior with `luet upgrade --sync`

pkg/package/package.go

@@ -107,6 +107,9 @@ type Package interface {
 	HumanReadableString() string
 	HashFingerprint(string) string
 
+	SetBuildTimestamp(s string)
+	GetBuildTimestamp() string
+
 	Clone() Package
 }
 
@@ -170,9 +173,10 @@ type DefaultPackage struct {
 	// Path is set only internally when tree is loaded from disk
 	Path string `json:"path,omitempty"`
 
-	Description string   `json:"description,omitempty"`
-	Uri         []string `json:"uri,omitempty"`
-	License     string   `json:"license,omitempty"`
+	Description    string   `json:"description,omitempty"`
+	Uri            []string `json:"uri,omitempty"`
+	License        string   `json:"license,omitempty"`
+	BuildTimestamp string   `json:"buildtimestamp,omitempty"`
 
 	Labels map[string]string `json:"labels,omitempty"` // Affects YAML field names too.
 }
@@ -207,7 +211,7 @@ func (p *DefaultPackage) GetFingerPrint() string {
 
 func (p *DefaultPackage) HashFingerprint(salt string) string {
 	h := md5.New()
-	io.WriteString(h, fmt.Sprintf("%s-%s",p.GetFingerPrint(),salt))
+	io.WriteString(h, fmt.Sprintf("%s-%s", p.GetFingerPrint(), salt))
 	return fmt.Sprintf("%x", h.Sum(nil))
 }
 
@@ -229,6 +233,16 @@ func (p *DefaultPackage) GetPackageName() string {
 	return fmt.Sprintf("%s-%s", p.Name, p.Category)
 }
 
+// GetBuildTimestamp returns the package build timestamp
+func (p *DefaultPackage) GetBuildTimestamp() string {
+	return p.BuildTimestamp
+}
+
+// SetBuildTimestamp sets the package Build timestamp
+func (p *DefaultPackage) SetBuildTimestamp(s string) {
+	p.BuildTimestamp = s
+}
+
 // GetPath returns the path where the definition file was found
 func (p *DefaultPackage) GetPath() string {
 	return p.Path