Don't warn about organization prefix when operating on a git source

I have a module called `ntp`, which I'm pulling from a git repo. Without this patch, and just with this in my `Modulefile`:

```ruby
mod 'ntp', :git => 'git://github.com/puppetlabs/puppetlabs-ntp.git'
```

will result in this warning:

```
Invalid module name 'ntp', you should qualify it with 'ORGANIZATION/ntp' for resolution to work correctly
```

The change in this PR will prevent that warning when operating on git sources.