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>

This while (partNum<myNumOfParts) loop, doesn't look right..

It looks like it cycles through all parts of the logical file, and will result in only the last one being opened/read.. (?)
You won't notice reading hthor files, which always have 1 part, but if hthor is reading thor files with >1 part, then it's going to skip all parts bar last I think.