feat: Add R cataloger (#1790)

Add a cataloger that detects installed R packages by looking for DESCRIPTION
files. The base R package is now picked up in coverageImage tests in
test/cli/packages_cmd_test.go, so increment expected package counts for the
tests that use that image.

Signed-off-by: Will Murphy <will.murphy@anchore.com>

syft/formats/common/spdxhelpers/source_info.go

@@ -52,6 +52,8 @@ func SourceInfo(p pkg.Package) string {
 		answer = "acquired package info from linux kernel module files"
 	case pkg.NixPkg:
 		answer = "acquired package info from nix store path"
+	case pkg.Rpkg:
+		answer = "acquired package info from R-package DESCRIPTION file"
 	default:
 		answer = "acquired package info from the following paths"
 	}