scm: add support for mirrors

[jkloetzke]

This should support at least the following use cases:

• Setting up some preferred mirror location(s) that should be used before the actual location in the recipe. This is useful for corporate setups where an internal mirror absorbs most of the traffic.
• Adding fallback locations in case the primary location is unavailable.

Mirrors should probably only be used for deterministic SCMs. Otherwise the mirrors might not hold the same data.

[mahaase]

just a note, currently it is possible to overwrite the GNU_MIRROR environment variable in the project default.yaml. that isn't a mirror-system, but it's cool! ;)