if a composer.json is present and a PHP version requirement is
configured, we set the php version to the minimal PHP version that
satisfies the composer requirement.
Additionally, this adds a `phpVersion` attribute to the <psalm> tag. If
that's set, it takes precedence over what has been detected in
composer.json.
And finally, the --php-version command line flag continues to work and
takes precedence over the setting in the <psalm> tag
this fixes#2628
