Add dependencies feature for package type

Previous to this commit, package providers could not provide dependency
information for installed packages on the node.  This commit introduces
a new feature, dependencies, that providers can implement.  The
'dependencies' feature is a read only property meaning it can not be
used in a manifest.  To implement the feature, a package provider only
has to declare it implements the feature by adding a line to the
provider: `has_feature :dependencies`. In addition, the `dependencies`
method must be implemented and must return an array with each
array element being a string that contains the name of the package that
is a direct dependency of the package resource.

Futher, this commit implements the dependencies feature in the apt
package provider.
Add spec tests for package dependencies feature

This commit adds spec tests for the package type's dependencies feature