This fixes a high frequency N+1 issue:

`RoutableActions#find_routable!` is used across many controllers to
retrieve e.g. the Project or Namespace by path. The `#find_routable!`
method calls `#ensure_canonical_path` which in turn retrieves
`#full_path` from the given Routable.

This in turn triggers a lookup on `routes`, leading to a high frequency
of these queries:

```sql
SELECT  "routes".* FROM "routes" WHERE "routes"."source_id" = $1 AND
"routes"."source_type" = $2 LIMIT $3
```

This is unnecessary as we already join `routes` in
`Routable#find_by_full_path` anyways.