optimize fetch to use local download

if source is already downloaded and can be verified, skip downloading again.

Author: h0tw1r3

lib/vanagon/component/source/http.rb

@@ -82,6 +82,13 @@ def initialize(url, sum:, workdir:, sum_type:, **options)
         # Download the source from the url specified. Sets the full path to the
         # file as @file and the @extension for the file as a side effect.
         def fetch
+          @file = File.basename(URI.parse(@url).path)
+          begin
+            return if verify
+          rescue RuntimeError, Errno::ENOENT
+            remove_instance_variable(:@file)
+          end
+
           @file = download(@url)
         end