More distinctive url scheme for tarball mirror and/or option to specify full fallback urls in recipes

Problem

openjpeg and openh264 both have source tarballs names v2.5.0.tar.gz (for example) and the way our tarball mirror works (https://gstreamer.freedesktop.org/src/mirror/${tarball_file_name)) that means we can only mirror one or the other.

Possible solutions

1. Different mirror scheme, e.g. having project/recipe name subdirectories, or having hash aliases linking to the actual tarball.
2. Allow recipes to specify full fallback URLs (has added benefit to allow other mirrors too)