MDL-34290 repository API: do not confuse source and reference

MDL-34290 repository API: do not confuse source and reference

make sure that repository function get_file_source_info receives as argument the source of the file,
and get_file receives a reference;
reference is a value of DB field files_reference.reference and result of get_file_reference(source).
Fix dropbox as the only repository that have different values in those fields;
also added information about user in dropbox reference and original