Add retrieve to compilation spec

It allows to copy build artifact during buildtime. In this way
artifacts between different seed images can be shared

```
retrieve:
- a-test-1.0.package.*
- https://...
```

They will be available under WORKDIR

pkg/compiler/interface.go

@@ -154,6 +154,9 @@ type CompilationSpec interface {
 
 	GetSourceAssertion() solver.PackagesAssertions
 	SetSourceAssertion(as solver.PackagesAssertions)
+
+	GetRetrieve() []string
+	CopyRetrieves(dest string) error
 }
 
 type CompilationSpecs interface {