Support moduledir with relative path

When using the moduledir directive in a Puppetfile, the path was assumed to
be an absolute path, even if the path did not start with a slash. In that case, the
directory was relative to the directory from where the user ran the `r10k` command.

Now when the moduledir path is relative, the path is joined with the Puppetfile basedir.
When the moduledir path is absolute, the behavior is the same as before.