Fixes #57
Relates #114
Relates MNG-6802, MNG-5612

A profile activation file is being interpolated during flatten-maven-plugin run.

Example

<profile>
    <id>dockerfile-update</id>
    <activation>
        <file>
            <exists>Dockerfile</exists> <!-- problem here -->
        </file>
    </activation>
</profile>

Result

<profile>
    <id>dockerfile-update</id>
    <activation>
        <file>
            <exists>aboslute/path/resolved/on/building/machine/Dockerfile</exists>
        </file>
    </activation>
</profile>

The root of the problem in FileProfileActivator.isActive()
method.

It is called in DefaultModelBuilder.build().
Which happens here.

That's why rawModel has already interpolated the file path.

It means when flatten-maven-plugin calls this.project.getOriginalModel()
it returns already interpolated activation file.

As a solution, we can load a raw model directly from the pom.xml.

That's why I introduced method getOriginalModel() which loads
Model from the proper pom.xml.

During troubleshooting, I added a unit-ish test FlattenMojoTest
to be able to debug.
That's why I added maven-plugin-testing-harness and maven-compat
dependencies. I can remove the test if you wish.

@hohwille @rfscholte could you take a look.

If you agree that FileProfileActivator.isActive() has a bug because it updates the input parameter profile, I'll be happy to fix it.