Given that the file potentially gets installed, it needs to be present during build time already. Checking during the import is too late.
So far the data directory constant has pointed to the source directory to be usable with different subdirectories. Now only the data subdirectory itself is being used with the constant, so point to the directory directly.
Also introduces a separate constant for the sql directory, so that it can be put separately from the rest of the data if required.
