(PUP-3678) skip non-directories on the module path

This avoids pointless warnings about invalid modules that started with puppetlabs/puppet#3261.
I don't think this is sufficient to fix the problem.  This change doesn't skip non-directories, it only skips files or directories that start with `.`.

Consider an environment directory that has a top level README:

```
$ bundle exec puppet apply -v --noop foo.pp
Error: Invalid module name README.md; module names must be alphanumeric (plus '-'), not 'README.md'
Error: Invalid module name README.md; module names must be alphanumeric (plus '-'), not 'README.md'
Info: Loading facts
Notice: Compiled catalog for mccune.corp.puppetlabs.net in environment production in 0.03 seconds
Info: Applying configuration version '1416333397'
Notice: /Stage[main]/Main/Tacacs_server_group[TAC-GR2]/servers: current_value ['1.2.3.5/50', '1.2.3.6/49', '1.2.3.6/50'], should be ['1.2.3.5/50'] (noop)
Notice: /Stage[main]/Main/Tacacs_server_group[TAC-GR-TRANS]/ensure: current_value present, should be absent (noop)
Notice: Class[Main]: Would have triggered 'refresh' from 2 events
Notice: Stage[main]: Would have triggered 'refresh' from 1 events
Notice: Applied catalog in 0.48 seconds
```

My suggestion is to replace this change with something like:

```ruby
fullpath = Pathname.new(path) + name
next unless fullpath.directory?
```