Expose which search strategy produced each result

query_packages now assigns @search_sources - a hash mapping package
ids to the matching strategy that put them into the results: :exact,
:name_prefix, :name_contains, :semantic or :fulltext. The grid and
list views render a small badge with a humanized label per result,
making the search tiers observable. semantic_results() was folded
into search_packages() so each branch tags its own source. Adds the
rails-controller-testing gem for assigns() in tests.

app/helpers/packages_helper.rb

@@ -1,4 +1,16 @@
 module PackagesHelper
+  # Human readable label for the search strategy that put a package
+  # into the search results (see PackagesController#search_packages)
+  def search_source_label(source)
+    {
+      exact: 'exact match',
+      name_prefix: 'name match',
+      name_contains: 'name match',
+      semantic: 'similar',
+      fulltext: 'full-text'
+    }[source]
+  end
+
   # Return the description and/or the version of the package
   def screenshot_caption(screenshot)
     str = []