[package] Make relay-mock-network-layer a peer dependency.
[package] Make relay-mock-network-layer a peer dependency.

This is useful for dev when projects want to make use of
`StorybookRouter`, but that doesn’t (yet) apply to all consumers.

artsy/reaction,xtina-starr/reaction,artsy/reaction,artsy/reaction,xtina-starr/reaction,artsy/reaction-force,xtina-starr/reaction,xtina-starr/reaction,artsy/reaction-force