(PUP-3396) Make it possible to add a module to Loaders

This makes it possible to add a module after the Loaders have been
initialized with modules from a module path. To add a module
call Loaders.add_module(Puppet::Module.new(name, path)). This should be
done after the Compiler has created the Loaders, and preferrably before
any code loading has started to take place.
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