Better strategy to find source file's path

In ```CloverXmlCoverageCollector``` the file path is taken from the ```name``` attribute of each ```file``` element in the clover report but the clover spec (which I got from Atlassian clover-ant) states that:
* the ```name``` attribute only contains the filename
* the ```path``` attribute contains the full absolute path to the file

Here is an excerpt from the clover's XSD:

```xsd
<xs:element name="file">
    <xs:annotation>
        <xs:documentation>
            File metrics.
            @name - the file name e.g. Foo.java or Bar.groovy
            @path - the filesystem-specific original path to the file e.g. c:\path\to\Bar.groovy
        </xs:documentation>
    </xs:annotation>
    <xs:complexType>
        <xs:sequence>
            <xs:element name="metrics" type="fileMetrics"/>
            <xs:element maxOccurs="unbounded" ref="class"/>
            <xs:element minOccurs="0" maxOccurs="unbounded" ref="line"/>
         </xs:sequence>
         <xs:attribute name="name" use="required" type="xs:NCName"/>
         <xs:attribute name="path" use="required"/>
    </xs:complexType>
</xs:element>
```

So I think it would be better to use this attribute when trying to find the full path but this would probably introduce a BC break in some cases.
With this PR, I tried to add an alternative : if the ```path``` attribute is provided, then we use it, otherwise, we fallback on the ```name``` attribute as it is the actual behavior and we should keep it.