Allow PhpDocumentor2 path to be found if installed via composer

If PhpDocumentor 2 is installed via composer it's unable to find to correct include path on system that's case sensitive.

Composer creates following directory structure **vendor/phpdocumentor/phpdocumentor** but phing expects it to be **vendor/phpdocumentor/phpDocumentor**.
This PR solves this as it will try first to find the one with the case sensitive path afterwards the lowercase one.

Example build file:
```xml
<?xml version="1.0" encoding="UTF-8" ?>
<project name="myapp" default="help" basedir=".">
    <property name="basedir" value="${project.basedir}" />
    <property name="builddir" value="${basedir}/build" />
    <property name="source" value="${basedir}" />

    <target name="phpdoc">
        <includepath classpath="${basedir}/vendor/phpdocumentor" />
        <phpdoc2 title="API Documentation" destdir="${builddir}/docs" template="zend">
            <fileset dir="${source}">
                <patternset refid="defaultSet" />
            </fileset>
        </phpdoc2>
    </target>
```