internal/pkg: block on implementation entry

This avoids blocking while not in Cure method of the implementation.

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

internal/pkg/pkg_test.go

@@ -283,7 +283,7 @@ func checkWithCache(t *testing.T, testCases []cacheTestCase) {
 			msg.SwapVerbose(testing.Verbose())
 
 			var scrubFunc func() error // scrub after hashing
-			if c, err := pkg.Open(t.Context(), msg, 0, base); err != nil {
+			if c, err := pkg.Open(t.Context(), msg, 1<<4, base); err != nil {
 				t.Fatalf("Open: error = %v", err)
 			} else {
 				t.Cleanup(c.Close)
@@ -561,6 +561,10 @@ func TestCache(t *testing.T) {
 							stub.UniqueError
 						}{UniqueError: 0xbad},
 					)},
+
+					cure: func(f *pkg.FContext) error {
+						panic("attempting to cure impossible artifact")
+					},
 				}, nil, pkg.Checksum{}, &pkg.DependencyCureError{
 					{
 						Ident: unique.Make(pkg.ID{0xff, 3}),