(PUP-3121) Lookup dependencies by forge name

Previously, if a module had a dependency named "puppetlabs-stdlib", it
would call `Environment.module_by_forge_name` with that name, but that
method expects names to be of the form `author/name`.

This commit translates the name prior to making the call.
I think we need to resolve dependencies using both `'-'` and `'/'`