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.

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package Bla;

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class Customer {

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    // attribute id

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    public int getId() {

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        return id;

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    }

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    public void setId(int value) {

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        id = value;

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    }

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    private int id;

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    // attribute name

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    public String getName() {

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        return name;

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    }

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    public void setName(String value) {

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        name = value;

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    }

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    private String name;

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}