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.

package Bla;

class Customer {
    // attribute id

    public int getId() {
        return id;
    }

    public void setId(int value) {
        id = value;
    }

    private int id;

    // attribute name

    public String getName() {
        return name;
    }

    public void setName(String value) {
        name = value;
    }

    private String name;
}