Break puppet dependency for metadata reading

[ringods]

Librarian-puppet only has a development dependency on puppet, but the code for processing Git sources in a Puppetfile still uses Puppet code directly to read the Puppet metadata file to figure out the module version and it's dependencies.

If the puppet gem isn't a dependency, then we need to get rid of the Puppet metadata reader in the Git code.

This change request is directly related to #134.

Should we duplicate the metadata reading code in here (Apache2 to MIT License?), or ask Puppetlabs to break it of in a separate gem? @joshcooper, can you give the Puppetlabs perspective on this?

[ringods]

I submitted an Improvement request in the new PuppetLabs JIRA to split out the Modulefile handling in a separate gem:

https://tickets.puppetlabs.com/browse/PUP-1188

With this functionality as a separate gem, it would solve the Puppet code dependency that is now still in place:

https://github.com/rodjek/librarian-puppet/blob/master/lib/librarian/puppet/source/git.rb#L74