Fix #4349 - Parsing with ignoreimport=true was always loading site.pp

With the type collection refactoring, when accessing a fresh collection
puppet tries to import the site.pp manifest (perfrom_initial_import).
In the case of puppetdoc, we are parsing site.pp by ourselves, so we
ended parsing it twice, resulting in an "import loop detected" error.

Signed-off-by: Brice Figureau <brice-puppet@daysofwonder.com>