Support alpm PURL type (#8898)

As specified on https://github.com/package-url/purl-spec/blob/main/types-doc/alpm-definition.md.

Author: Pietrucci-Blacher

_plugins/identifier-to-url.rb

@@ -207,4 +207,8 @@ def _build_swid_url(purl)
   def _build_generic_url(purl)
     return nil # valid, but don't know how to generate this kind of URL
   end
+
+  def _build_alpm_url(purl)
+    return "https://archlinux.org/packages/?q=#{purl.name}"
+  end
 end