Introduce relationships as first-class objects (#607)

* migrate pkg.ID and pkg.Relationship to artifact package

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

* return relationships from tasks

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

* fix more tests

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

* add artifact.Identifiable by Identity() method

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

* fix linting

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

* remove catalog ID assignment

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

* adjust spdx helpers to use copy of packages

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

* stabilize package ID relative to encode-decode format cycles

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

* rename Identity() to ID()

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

* use zero value for nils in ID generation

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

* enable source.Location to be identifiable

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

* hoist up package relationship discovery to analysis stage

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

* update ownership-by-file-overlap relationship description

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

* add test reminders to put new relationships under test

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

* adjust PHP composer.lock parser function to return relationships

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

syft/sbom/sbom.go

@@ -1,6 +1,7 @@
 package sbom
 
 import (
+	"github.com/anchore/syft/syft/artifact"
 	"github.com/anchore/syft/syft/distro"
 	"github.com/anchore/syft/syft/file"
 	"github.com/anchore/syft/syft/pkg"
@@ -8,8 +9,9 @@ import (
 )
 
 type SBOM struct {
-	Artifacts Artifacts
-	Source    source.Metadata
+	Artifacts     Artifacts
+	Relationships []artifact.Relationship
+	Source        source.Metadata
 }
 
 type Artifacts struct {