*: migrate consumers from nox-xdge / dune-internal xdg to nox-xdg{,.eio}

Sweep of every consumer that previously depended on either the
standalone [nox-xdge] package (the Eio wrapper) or dune's internal
[xdg] library. Both are gone after the previous commit; consumers
now point at the merged [nox-xdg] package's two libraries:

- Code that used the Eio wrapper ([Xdge.v fs app_name],
[Xdge.config_dir], etc.) -> [Xdg_eio.foo], library [nox-xdg.eio].
- Code that used the pure spec ([Xdg.create ~env ()],
[Xdg.cache_dir]) -> unchanged module name [Xdg], library
[nox-xdg].

Touched packages:

- gauth, gdocs, gsheets, gslides -- the Google credential stack;
[Gauth.Local_store] uses [Xdg_eio] internally.
- ocaml-requests, ocaml-cookie -- HTTP client and cookie jar;
[Requests.t] takes an [?xdg:Xdg_eio.t] for cookie persistence.
- ocaml-agent, ocaml-atp, ocaml-slack, ocaml-linkedin, monopam,
uniboot -- CLIs that compute their own config / cache / state
paths via the Eio wrapper.
- ocaml-oci -- uses the pure spec ([Xdg.cache_dir]) for OCI image
layout; migrated from dune-internal [xdg] to [nox-xdg].

Mechanical changes per package:

- [dune-project]: opam dep [nox-xdge] -> [nox-xdg].
- [lib/dune] and [bin/dune]: library reference [nox-xdge] ->
[nox-xdg.eio] (or [nox-xdg] for the rare pure-spec consumer).
- [.ml] / [.mli]: [Xdge] -> [Xdg_eio].

Generated [.opam] files updated by the dune build accordingly.