add test coverage for python pacakge cataloger and update catalog interface

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

test/integration/pkg_coverage_test.go

@@ -68,6 +68,7 @@ func TestPkgCoverageImage(t *testing.T) {
 			}
 
 			if pkgCount != len(c.pkgInfo) {
+				t.Logf("Discovered packages of type %+v", c.pkgType)
 				for a := range catalog.Enumerate(c.pkgType) {
 					t.Log("   ", a)
 				}

test/integration/test-fixtures/image-pkg-coverage/python/dist-info/RECORD

@@ -0,0 +1,5 @@
+../../../bin/pygmentize,sha256=dDhv_U2jiCpmFQwIRHpFRLAHUO4R1jIJPEvT_QYTFp8,220
+Pygments-2.6.1.dist-info/AUTHORS,sha256=PVpa2_Oku6BGuiUvutvuPnWGpzxqFy2I8-NIrqCvqUY,8449
+Pygments-2.6.1.dist-info/RECORD,,
+pygments/__pycache__/__init__.cpython-38.pyc,,
+pygments/util.py,sha256=586xXHiJGGZxqk5PMBu3vBhE68DLuAe5MBARWrSPGxA,10778