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.

module Bla {
    struct Blub {
        long a;
        string b;
    };

    interface Blubber {
        Blub getBlub();
    };

    typedef My::Complicated::Reachable::Thing Thing;

    interface Irgendwas {
        struct Bla {

        };

        readonly attribute Thing b;
    };
};