RFE: native support for vpath builds

[pmatilai]

With the new rpm-controlled %builddir from #2078, we now have a safe playground for all sorts of things that didn't have a place to go before. One such item is vpath (aka out of tree) builds, whereas rpm traditionally always assumes build happens inside the source tree. What once was the defacto thing is nowadays frowned upon (for good reasons), and the once fancy-pants vpath build is closer to the norm.

We can never change the default %setup behavior without breaking tonnes of packages, but we should support the vpath build natively as an opt-in: unpack sources as usual but create and change to an empty directory fully outside the source tree for the build. This could be something new Buildsystem: implementations might want to take advantage of, for example.

To go with this, the source-tree could be made read-only after %prep.