FFAP support

[swsnr]

We should support ffap, to find the declaration of the class or type at point.

Shouldn't be too difficult, since Puppet has this super-easy autoload hierarchy, which nicely maps resource names to file names. Also, there's no scope import, so names in puppet are always fully qualified.

So essentially:

1. Find the first class or type definition in the buffer
2. Split it by :: to get the file name hierarchy, and traverse upwards to find the top-level module directory
3. Split the name at point by ::, and traverse downwards from the top level module directory to the target file
4. Visit the target file and search for the matching class or type definition

For use cases with multiple module directories, we should add a new option puppet-module-path, as a list of module directories for Puppet. ffap would look for the target file in all these directories additionally.