port ruby cataloger to new generic cataloger pattern (#1322)

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

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

syft/pkg/cataloger/ruby/package_test.go

@@ -0,0 +1,31 @@
+package ruby
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+)
+
+func Test_packageURL(t *testing.T) {
+	tests := []struct {
+		name     string
+		version  string
+		expected string
+	}{
+		{
+			name:     "p",
+			version:  "v",
+			expected: "pkg:gem/p@v",
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(fmt.Sprintf("%s@%s", test.name, test.version), func(t *testing.T) {
+			actual := packageURL(test.name, test.version)
+			if diff := cmp.Diff(test.expected, actual); diff != "" {
+				t.Errorf("unexpected packageURL (-want +got):\n%s", diff)
+			}
+		})
+	}
+}