Split manifests into one file per class/definition.

This is to better support use of the Puppet autoloader, which is
required from Puppet 4 and onwards.  We also remove the import
statements that were here, as they disappeared in Puppet 4.

Exception is internal helpers that are only used by its direct
"parent"; e.g. apache::foo::helper, used only by apache::foo, can
still reside in foo.pp.