Prefix all blacksun packages with nox-

Renames 35 packages to make blacksun forks distinguishable from their
opam-repository upstreams. Module names (Git.x, Tls.x, ...) stay bare;
opam package names and dune (public_name) findlib references move to
nox-X. After this commit, zero local package names overlap with
opam-repository.

Renamed:
- nox-git, nox-irmin
- nox-crypto, nox-crypto-pk, nox-crypto-rng, nox-crypto-ec
- nox-tls, nox-tls-eio, nox-tar, nox-tar-eio, nox-tty, nox-tty-eio
- nox-arp, nox-ca-certs, nox-cbor, nox-cookie, nox-crc, nox-csv
- nox-gpt, nox-hkdf, nox-http, nox-jwt, nox-kdf, nox-loc
- nox-memtrace, nox-pds, nox-sexp, nox-slack, nox-toml
- nox-websocket, nox-x509, nox-xdge, nox-yaml

Also drops orphan tar-mirage and tar-unix opam templates that had no
matching package stanza.