Make kpm support multiple kinds of local repositories as package sources

[davidfowl]

Today kpm supports the set of package sources that nuget supports for remote repositories (where packages come from):

1. NuGet v2 odata feeds
2. A flat list of nupkg files
3. A nuget style expanded local repository ({id}.{version}/{id}.{version}.nupkg)

We need to add support for the kpm restore image on disk as a source:

'{id}/{version}/{id}.{version}.nupkg'