You need to call cabal-dev add-source on P1 again to copy over the sdist, then do a cabal-dev install.

See notes under "Using a sandbox-local Hackage" on https://github.com/creswick/cabal-de

With the new cabal sandboxing (due in 1.18) this won't be necessary as we create a link to the repo, instead of installing a copy. We will rebuild the linked repo as needed.