spec/manifest: clarify relationship between urls and foreign layers
spec/manifest: clarify relationship between urls and foreign layers

Previously, the specificiation incorrectly bound the fates of `urls` and
foreign layers. These are complementary but unrelated features, in that
the `urls` field may be populated for layers that aren't foreign. The
type of the layer only dictates the push behavior of the layer, rather
than involving where it came from.

For example, one may pull a foreign layer from a registry, but they may
not push it back to another registry. Conversely, a layer that has no
restrictions on push/pull behavior may be fetched via `urls` entries.

Signed-off-by: Stephen J Day <63efd86fa22882ae89b6027b22a36e4db1f4214d@docker.com>

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