Check that Puppet[:manifest] exists before importing it

Currently it is checked if `Puppet[:modulepath]` exists before importing `Puppet[:manifest]`. This causes problems if `Puppet[:modulepath]` contains several paths (separated by `:`).

Handling multiple module paths correctly probably requires changes in several places, but while I started to look into it, I stumbled across this line, which didn't make sense to me.

Please feel free to reject this pull request if there is a good reason to check for the existence of `Puppet[:modulepath]` that I didn't comprehend.