generate_*: improve includes to be more consistent

This is just stylistic change. It removes dirname and instead relies on
string operation. There should be no way of having in $0 anything else
other than some valid path. That is last part has to be at least
/name_of_file. This means that we do not have to rely on dirname and we
can safely just remove anything after last slash (including that slash).
We user dirname only for paths in sub-directories.