Investigate if git submodules can be included

[asgrim]

At the moment, PIE downloads the ZIP archive, which doesn't include git submodules. mongodb does use submodules for the C libs to be included.

The .gitmodules file is present, however, the revision information is kept in .git which isn't included in the Zip archive.

[asgrim]

I think this may now be possible with the following changes:

1. we flag some way of forcing this extension to always be from source (i.e. add the ->setPreferSource() call in \Php\Pie\ComposerIntegration\ComposerIntegrationHandler). This could either be a special case we write into PIE itself, or probably better, add a composer.json configuration that Mongo (and other extensions with git modules) can specify. This will ask Composer to download the source with Git instead of the ZIP archive
2. we add a step into \Php\Pie\Building\UnixBuild before the phpize to run git submodule update --init; this could either be based on the configuration mentioned, or maybe we could detect which git + existence of .gitmodule file + existence of .git path; a warning could be issued if .gitmodule exists but .git does not (indicating we cannot initialise submodules, since .git is missing).

I think add a composer.json config, something like this could work:

{
    "name": "somevendor/ourextension",
    ...
    "php-ext": {
        "has-git-submodules": true,
    }
}

What do you think folks @alcaeus / @GromNaN ?