(PUP-865) Add ability to parse a directory of manifests as initial import

This adds the ability to use a directory instead of a single manifest file. This behavior is needed because
the support for "import" is deprecated and will be removed in Puppet 4. Instead, all the files that should be "imported" are placed in a single directory and the directory is then given as the manifest (instead of just a single site.pp).

The files are parsed and evaluated in alphabetical order.