Make sure path/scm location is used during dependency resolution

This fixes two problems when using path and SCM locations:

  1. Custom cookbook on given location uses different dependencies

    For example if someone uses a local/scm cookbook with the same name/version available in the community cookbook site, the dependencies of the community cookbook will be added to the resolution graph. This change stops to populate the dependency graph with any cookbook dependencies from global sources if a local cookbook is already present. 

  2. Solve graph _only_ using the demanded location cookbook version

    If a location is specified, the cookbook constraint should be locked to that version automatically.

Fixes #975.