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>

internal/formats/syftjson/test-fixtures/snapshot/TestDirectoryPresenter.golden

@@ -1,7 +1,7 @@
 {
  "artifacts": [
   {
-   "id": "package-1-id",
+   "id": "cbf4f3077fc7deee",
    "name": "package-1",
    "version": "1.0.1",
    "type": "python",
@@ -36,7 +36,7 @@
    }
   },
   {
-   "id": "package-2-id",
+   "id": "1a39aadd9705c2b9",
    "name": "package-2",
    "version": "2.0.1",
    "type": "deb",

internal/formats/syftjson/test-fixtures/snapshot/TestImagePresenter.golden

@@ -1,7 +1,7 @@
 {
  "artifacts": [
   {
-   "id": "package-1-id",
+   "id": "d1d433485a31ed07",
    "name": "package-1",
    "version": "1.0.1",
    "type": "python",
@@ -32,7 +32,7 @@
    }
   },
   {
-   "id": "package-2-id",
+   "id": "2db629ca48fa6786",
    "name": "package-2",
    "version": "2.0.1",
    "type": "deb",