Populate Files and Relationship fields for spdx-json output (#507)

* update spdx22 Document model to include relationships field

Signed-off-by: Christopher Angelo Phillips <christopher.phillips@anchore.com>

* update document and relationship to match current JSON spec
https://github.com/spdx/spdx-spec/blob/development/v2.2.1/schemas/spdx-schema.json
https://github.com/spdx/spdx-spec/pull/528
https://github.com/spdx/spdx-spec/pull/528#issuecomment-904180177

Signed-off-by: Christopher Angelo Phillips <christopher.phillips@anchore.com>

* update File struct based on SPDX schema

Required fields:
[ "SPDXID", "fileName", "copyrightText", "licenseConcluded" ]
Signed-off-by: Christopher Angelo Phillips <christopher.phillips@anchore.com>

syft/pkg/file_owner.go

@@ -1,5 +1,9 @@
 package pkg
 
-type fileOwner interface {
-	ownedFiles() []string
+// FileOwner is the interface that wraps OwnedFiles method.
+//
+// OwnedFiles returns a list of files that a piece of
+// package Metadata indicates are owned by the package.
+type FileOwner interface {
+	OwnedFiles() []string
 }