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

tests/fixtures/retrieve-integration/a/build.yaml

@@ -0,0 +1,3 @@
+image: "alpine"
+steps:
+  - echo a > /a

tests/fixtures/retrieve-integration/a/definition.yaml

@@ -0,0 +1,3 @@
+name: "a"
+version: "1.0"
+category: "test"

tests/fixtures/retrieve-integration/b/build.yaml

@@ -0,0 +1,10 @@
+
+steps:
+- tar xvf a-test-1.0.package.* -C ./
+- mv a /b
+requires:
+- name: "a"
+  version: "1.0"
+  category: "test"
+retrieve:
+- a-test-1.0.package.*

tests/fixtures/retrieve-integration/b/definition.yaml

@@ -0,0 +1,7 @@
+name: "b"
+version: "1.0"
+category: "test"
+requires:
+- name: "a"
+  version: "1.0"
+  category: "test"