internal/pkg: deduplicate DCE by ident

This eliminates edge cases where target artifacts do not compare equal.

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

internal/pkg/pkg_test.go

@@ -1515,6 +1515,8 @@ func TestDependencyCureError(t *testing.T) {
 	makeIdent := func(ident ...byte) pkg.Artifact {
 		var a overrideIdent
 		copy(a.id[:], ident)
+		// does not compare equal
+		a.TrivialArtifact = new(stubArtifact)
 		return a
 	}