Move pin-depends to the packages that need them

opam propagates pin-depends through the dependency graph during solve,
but only if the pin is declared in a package that's part of the install
set. Anil hits "scitt is solve failing due to missing tw" because none
of the packages in his install chain (scitt -> nox-irmin -> tw) declare
the tw pin — only root.opam.template did, and root isn't installed.

Move each pin to the user-facing package that consumes it:

- tw.dev -> nox-irmin (irmin/lib/ui uses tw)
- wire.dev -> tm (ocaml-tm/lib uses wire)

Drop cascade.dev from root entirely — no consumer in the tree. Keep
alcobar.dev (66 with-test consumers; only relevant for dev installs)
and helix.dev (only globe's private demo) in root.