Build: make build-deps installs ledgerwallet-tezos

Context

ledgerwallet-tezos was removed from vendors and thus became truly optional, even for devs and for users that use make build-deps && make. This is a breaking change. Instead of documenting it as a breaking change, this MR restores the previous behavior by making sure that make build-deps installs ledgerwallet-tezos. To this end, it introduces a dummy package that has ledgerwallet-tezos as a dependency so that opam install --deps-only in install_build_deps.raw.sh installs ledgerwallet-tezos.

Manually testing the MR

opam remove --criteria=-changed ledgerwallet-tezos
make build-deps

Checklist

• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR