Prevent the importContent step from being run over and over again 

When you write your own package that installs p.a.contenttypes via dependency in metadata.xml, the importContent GS step is run multiple times when the package gets installed.

This might not be such a problem in general, but it will mess up any pre-existing language settings, since `_set_language_settings` is written in a way that does not consider existing settings (i.e. it will remove all supported languages except the default).

By using the time-honoured text file check, this can be prevented.