pageserver: respect has_relmap_file in collect_keyspace (#8276)

## Problem

Rarely, a dbdir entry can exist with no `relmap_file_key` data. This
causes compaction to fail, because it assumes that if the database
exists, then so does the relmap file.

Basebackup already handled this using a boolean to record whether such a
key exists, but `collect_keyspace` didn't.

## Summary of changes

- Respect the flag for whether a relfilemap exists in collect_keyspace
- The reproducer for this issue will merge separately in
https://github.com/neondatabase/neon/pull/8232

pageserver/src/pgdatadir_mapping.rs

@@ -854,13 +854,14 @@ impl Timeline {
         result.add_key(DBDIR_KEY);
 
         // Fetch list of database dirs and iterate them
-        let buf = self.get(DBDIR_KEY, lsn, ctx).await?;
-        let dbdir = DbDirectory::des(&buf)?;
+        let dbdir = self.list_dbdirs(lsn, ctx).await?;
+        let mut dbs: Vec<((Oid, Oid), bool)> = dbdir.into_iter().collect();
 
-        let mut dbs: Vec<(Oid, Oid)> = dbdir.dbdirs.keys().cloned().collect();
-        dbs.sort_unstable();
-        for (spcnode, dbnode) in dbs {
-            result.add_key(relmap_file_key(spcnode, dbnode));
+        dbs.sort_unstable_by(|(k_a, _), (k_b, _)| k_a.cmp(k_b));
+        for ((spcnode, dbnode), has_relmap_file) in dbs {
+            if has_relmap_file {
+                result.add_key(relmap_file_key(spcnode, dbnode));
+            }
             result.add_key(rel_dir_to_key(spcnode, dbnode));
 
             let mut rels: Vec<RelTag> = self