Merge: nitcatalog: isolate package page into a subdirectory

HTML files are generated into a `/p/` subdirectory so they will not conflict with other generated files.

One other point is to have stable and final URL for packages in the catalog. eg http://nitlanguage.org/catalog/p/nitiwiki.html

Pull-Request: #1714
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>