internal/pkg: record cure faults

These are useful for troubleshooting. This change records them in a separate directory.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pkg/pkg_test.go

@@ -492,6 +492,12 @@ func checkWithCache(t *testing.T, testCases []cacheTestCase) {
 			if err := os.RemoveAll(base.Append("status").String()); err != nil {
 				t.Fatal(err)
 			}
+
+			// destroy empty fault directory
+			if err := os.Remove(base.Append("fault").String()); err != nil {
+				t.Fatal(err)
+			}
+
 			want := tc.want.hash()
 
 			var checksum pkg.Checksum