Previously we had the functionality to detect and raise warnings when undefined classes/functions were used in `libphutil` modules. This functionality was broken in D9864. Previously, `$symbols` was the raw output of `phutil_symbols.php`. After D9864, it's the processed output of `PhutilLibraryMapBuilder`, which no longer has the `need` key.
This issue was identified in T1806 (see T1806#11 and T1806#13).