pageserver: remove bare mgr::get_tenant, mgr::list_tenants (#7237)

## Problem

This is a refactor.

This PR was a precursor to a much smaller change
e5bd602dc1,
where as I was writing it I found that we were not far from getting rid
of the last non-deprecated code paths that use `mgr::` scoped functions
to get at the TenantManager state.

We're almost done cleaning this up as per
https://github.com/neondatabase/neon/issues/5796. The only significant
remaining mgr:: item is `get_active_tenant_with_timeout`, which is
page_service's path for fetching tenants.

## Summary of changes

- Remove the bool argument to get_attached_tenant_shard: this was almost
always false from API use cases, and in cases when it was true, it was
readily replacable with an explicit check of the returned tenant's
status.
- Rather than letting the timeline eviction task query any tenant it
likes via `mgr::`, pass an `Arc<Tenant>` into the task. This is still an
ugly circular reference, but should eventually go away: either when we
switch to exclusively using disk usage eviction, or when we change
metadata storage to avoid the need to imitate layer accesses.
- Convert all the mgr::get_tenant call sites to use
TenantManager::get_attached_tenant_shard
- Move list_tenants into TenantManager.

pageserver/src/disk_usage_eviction_task.rs

@@ -61,7 +61,6 @@ use crate::{
     metrics::disk_usage_based_eviction::METRICS,
     task_mgr::{self, TaskKind, BACKGROUND_RUNTIME},
     tenant::{
-        self,
         mgr::TenantManager,
         remote_timeline_client::LayerFileMetadata,
         secondary::SecondaryTenant,
@@ -814,8 +813,8 @@ async fn collect_eviction_candidates(
     const LOG_DURATION_THRESHOLD: std::time::Duration = std::time::Duration::from_secs(10);
 
     // get a snapshot of the list of tenants
-    let tenants = tenant::mgr::list_tenants()
-        .await
+    let tenants = tenant_manager
+        .list_tenants()
         .context("get list of tenants")?;
 
     // TODO: avoid listing every layer in every tenant: this loop can block the executor,
@@ -827,8 +826,12 @@ async fn collect_eviction_candidates(
         if cancel.is_cancelled() {
             return Ok(EvictionCandidates::Cancelled);
         }
-        let tenant = match tenant::mgr::get_tenant(tenant_id, true) {
-            Ok(tenant) => tenant,
+        let tenant = match tenant_manager.get_attached_tenant_shard(tenant_id) {
+            Ok(tenant) if tenant.is_active() => tenant,
+            Ok(_) => {
+                debug!(tenant_id=%tenant_id.tenant_id, shard_id=%tenant_id.shard_slug(), "Tenant shard is not active");
+                continue;
+            }
             Err(e) => {
                 // this can happen if tenant has lifecycle transition after we fetched it
                 debug!("failed to get tenant: {e:#}");