HPCC-10142 queryLogicalFilename not supported on hthor

Refactor the code
    - Separate the superfile processing and use nested iterators
    - Separate single file processing
    - Remove leading '~' char from logical file name.

Tested with issue10142.ecl

Signed-off-by: Attila Vamos <attila.vamos@gmail.com>

I think you're going to need this or similar on the non-superfile branch too, or some way of ensuring the expanded logicalfile name is set, e.g. if you're reading "myscope::file" (without a '~'), the file it reads is dependent on the workunit scope, typically hthor, so file read is "hthor::myscope::file"