Add archive_path helper to Tar and Url fetchers

Eventually I think we'll want this as part of the fetcher API generally.

Signed-off-by: Steven Danna <steve@chef.io>

lib/fetchers/tar.rb

@@ -19,6 +19,10 @@ module Fetchers
       new(target)
     end
 
+    def archive_path
+      target
+    end
+
     def initialize(target)
       @target = target
       @contents = {}

lib/fetchers/url.rb

@@ -103,6 +103,10 @@ module Fetchers
       @archive = self.class.download_archive(url, opts)
     end
 
+    def archive_path
+      @archive.path
+    end
+
     def url
       @target
     end

lib/inspec/dependencies/requirement.rb

@@ -108,7 +108,7 @@ module Inspec
       if fetcher.class == Fetchers::Local || @vendor_index.exists?(@name, source_url)
         local_path
       else
-        @vendor_index.add(@name, source_url, fetcher.archive.path)
+        @vendor_index.add(@name, source_url, fetcher.archive_path)
       end
     end