Add testing framework for metadata file validation on real data

Resolves #19 (closed)

There's also an README with an explanation on how to use it, please check whether everything works for you :)

While at it, a wrong spec in the BULIDINFO v2 spec has been fixed, which forbid buildtool versions without an architecture. Those are however allowed as packages with makepkg are semver only (e.g. 7.0.0)

This MR escalated a bit in that it also refactors quite a few things along the way that were apparently unergonomic while working with it. Those changes are, however, very well scoped into their own commits.