search manifests in manifests directory

[Dan33l]

The syntax check searches for all .pp files in the modules, which includes
.pp files in the spec tests of the modules like in spec/type_aliases. This can lead to errors like:

bundle exec rake syntax 
---> syntax:manifests
Attempt to redefine entity 'http://puppet.com/2016.1/runtime/type/XXX'. Originally set at file:///builds/XXX/manifests/network.pp?line=24&pos=1.