issue2551123 - validate indexer_language in configuration.py Was validated in backends/indexer_xapian.py which would throw an error on access rather than on start. Added validator function to CoreConfig class that runs after config.ini is read. At this time we have access to the indexer setting so can determine if xapian is actually going to be used. Moved test into test/test_config.py and pulled validation code from indexer_xapian.py and test/test_indexer.py.

# make this dir a package