Sort fixtures.yml, add --latest-versions flag, misc bug fixes

[nabertrand]

• Enable support for 'author-name' module syntax in the control repo Puppetfile
• Allow trailing comments in Puppetfile module lines
• Sort modules by name in .fixtures.yml
• Only include directories when adding .fixtures.yml symlinks
• Add a --latest-versions flag to cause .fixtures.yml to use the latest version of forge modules and the default branch of repository modules. The --latest-versions flag can be helpful for testing whether new versions of modules introduce spec test failures.

[rnelson0]

@nabertrand Nice job! Do you think we need any additional tests, specifically more complex or unsorted Puppetfiles, to ensure there's no regressions in the future?