preserve syft IDs on decode

Signed-off-by: Alex Goodman <alex.goodman@anchore.com>

internal/formats/syftjson/to_syft_model.go

@@ -177,8 +177,12 @@ func toSyftPackage(p model.Package, idAliases map[string]string) pkg.Package {
 		Metadata:     p.Metadata,
 	}
 
-	out.SetID()
+	// we don't know if this package ID is truely unique, however, we need to trust the user input in case there are
+	// external references to it. That is, we can't derive our own ID (using pkg.SetID()) since consumers won't
+	// be able to historically interact with data that references the IDs from the original SBOM document being decoded now.
+	out.OverrideID(artifact.ID(p.ID))
 
+	// this alias mapping is currently defunct, but could be useful in the future.
 	id := string(out.ID())
 	if id != p.ID {
 		idAliases[p.ID] = id

syft/pkg/package.go

@@ -28,6 +28,10 @@ type Package struct {
 	Metadata     interface{}        // additional data found while parsing the package source
 }
 
+func (p *Package) OverrideID(id artifact.ID) {
+	p.id = id
+}
+
 func (p *Package) SetID() {
 	id, err := artifact.IDByHash(p)
 	if err != nil {