docs: Include files are always searched at standard location, too.
This removes the need to specify the location where yml2 was unpacked or
installed as YML_PATH. So this now works:
YML_PATH=/foo ./yml2proc --xml2yml
The standard system location <prefix>/share/yml2 is now always appended to
the include search path. If this path does not exist, the directory one
above the `backend.py` file is added, allowing the same behaviour for
uninstalled yml2.
In the Makefile enforce the .ysl2 files to be searched in the current
directory first.
Move data files into the Python package.
The packe directory is now always searched for include files, after
all other directoreis given in $YML_PATH.