fdo#75304: Add test for external reference cache import from XLSX.

Change-Id: I93c4398b5673e1edaacfe4b68015000dfd224612
fdo#75304: Set external reference info to ScCompiler.

And remove this excessive const fetish which was unfortunately necessary to
get this to compile.  Setting the container const is good enough.

Change-Id: I2c6cc55a88643a68e065c4518cdf069c247f4f02