detect already installed packages on webpi source

This change ensures that packages using `source => webpi` do not get marked as created on every Puppet execution, by detecting they are already installed.

The parsing is a little crude but should work. I've submitted https://github.com/chocolatey/chocolatey/pull/521 to avoid the need to exclude Available Products at some future time.

Note that this doesn't list the packages in `self.instances`. I'm not sure what the impact of that is for the provider. The only way I could think of addressing that is to split webpi into a separate provider, where every operation uses `-source webpi`. This might make sense as a next step regardless, so that it can have correct capabilities (e.g., latest also doesn't make sense on webpi, and it is not versionable). Would that be a change you'd consider?