chore: Move authn into cmd/frontend

[eseliger]

They should not be used outside of cmd/frontend, so making it a frontend internal package.

While doing that, I realized that there is a coupling dependency between authz providers and auth (which is authN) providers: GitLab code host connections can do authz mapping via the usernames of another OIDC or SAML auth provider (https://sourcegraph.com/docs/admin/code_hosts/gitlab#administrator-sudo-level-access-token). It turns out this feature does not work anymore, since at least several releases, because we don't actually instantiate auth providers outside of cmd/frontend and thus the mapping will never find anything (auth providers don't explode when queried before init, unlike authz).
This only now became clear as I moved this code, and the dependency graph was broken, so that's a nice property of these cleanups I guess 😬
Since it doesn't seem to work for quite some time, I opted for removing it, and added a changelog entry about it. Not sure if that is sufficient, I raised a thread here: https://sourcegraph.slack.com/archives/C03K05FCRFH/p1721848436473209.
This would've prevented this change and needed more refactoring as unfortunately we cannot map an auth provider by the conf type to a record in the user_external_accounts table and need to actually instantiate it.

Test plan: Compiler doesn't complain, tests still pass.

Changelog

GitLab code host connections were able to sync permissions by mapping Sourcegraph users to GitLab users via the username property of an external OIDC or SAML provider that is shared across Sourcegraph and GitLab. This integration stopped working a long time ago, and it has been removed in this release.

[eseliger]

• searcher: Consolidate FetchTar and FetchTarPaths #64268
• chore: Move cmd/frontend/backend to internal #64261
• auth: Remove redirects to session package #64260
• chore: Cleanup more cross-cmd imports #64259
• oidc: Don't require to run refresh on provider init #64159
• chore: Remove unnecessary _ imports #64158
• chore: Move cmd/frontend/webhooks to cmd/frontend/internal #64157
• frontend: Consolidate remaining registry packages #64156
• frontend: Remove global conf server variable #64155
• chore: move internal/suspiciousnames to cmd/frontend/internal #64071
• chore: remove cmd/frontend/external redirection package #64070
• chore: move cmd/frontend/oneclickexport to cmd/frontend/internal/oneclickexport #64069
• chore: Simplify license routines #64068
• chore: Remove unused loghandlers package #64067
• chore: Deglobalize oneclickexporter instance #64066
• chore: move internal/highlight to cmd/frontend #64065
• chore: move internal/conf/validation to cmd/frontend #64064
• auth: Fix Found page response when redis is down #64063
• chore: Remove redis init side-effect of app.NewHandler #64062
• chore: Init userpasswd provider where we initialize every other authn provider #64061
• chore: Remove global externalURL watcher #64058
• cleanup: Move azure openai provider config validation to right place #64056
• chore: Don't pass nil context #64055
• chore: Remove cmd/frontend/external/app #64054
• licensecheck: Make proper goroutine #63655
• chore: Make return type more obvious #63651
• subrepo: More explicitly define supported hosts #63650
• Move internal/session into cmd/frontend/internal #63649
• chore: Move authn into cmd/frontend #63648 👈
• authz: Compute providers on the fly #64012
• authz: Drop requirement for installing authz providers in every service #63743
• main

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