Add portage support for Gentoo Linux (#1076)

Co-authored-by: Christopher Phillips <christopher.phillips@anchore.com>

internal/formats/common/spdxhelpers/source_info.go

@@ -37,6 +37,8 @@ func SourceInfo(p pkg.Package) string {
 		answer = "acquired package info from PHP composer manifest"
 	case pkg.ConanPkg:
 		answer = "acquired package info from conan manifest"
+	case pkg.PortagePkg:
+		answer = "acquired package info from portage DB"
 	default:
 		answer = "acquired package info from the following paths"
 	}

internal/formats/common/spdxhelpers/source_info_test.go

@@ -158,6 +158,14 @@ func Test_SourceInfo(t *testing.T) {
 				"from conan manifest",
 			},
 		},
+		{
+			input: pkg.Package{
+				Type: pkg.PortagePkg,
+			},
+			expected: []string{
+				"from portage DB",
+			},
+		},
 	}
 	var pkgTypes []pkg.Type
 	for _, test := range tests {