Move remotes to Suggests

[jennybc]

Follow-on to #2649 (and discussed there). But, more than a bit of faffing around with the previously-re-exported and now-deprecated remotes functions, this really requires removing/guarding all internal use of remotes. Since #2651 makes the opening move towards pak, we'll let the dust settle on various PRs that are already in flight, then come back to this.

[hadley]

Looks like this should be pretty straightforward:

• release() calls release_checks() calls check_dev_versions() calls remotes::standardise_dep(). Deprecated, so add check_installed()
• install_deps()/install_dev_deps() uses remotes::install_deps() and remotes::update_packages() in . Deprecated, so add check_installed()
• install(), fixed in Use pak::local_install_deps() in install() #2651
• Other install_ functions. All deprecated, so add check_installed()
• dev_sitrep(): replace remotes::package_dep() and remotes::dev_package_deps() with pak::pkg_deps() and pak::local_dev_deps().
• build_vignettes(). Deprecated, so add check_installed()

Also need to update "Conscious uncoupling" section in the readme.