(maint) Untie parser/files and module

The parser/files module was explicitly handling
Puppet::Module::InvalidName, forcing it to require puppet/module. These
errors are being handled by Puppet::Node::Environment at a lower layer,
and we no longer need to couple module and parser/files.

In addition, Pupper::Module#match_manifest ensures that init.pp is
always added if present, which means that explicitly searching for
init.pp would return an array with the init.pp file listed twice. Hence
the added uniq.