Build system refactoring

FYI: I'm intending on trying to do fairly large scale refactoring of
the build system (including w3ctestlib and apiclient) to try and end
up in somewhere saner, because at the moment the code is incredibly
difficult to navigate and frequently hard to understand, making it
hard to try and implement what we already have agreement on.

I realise that shepherd and the web wrapper for the drafts repository
also depend on these, but hopefully it won't be too hard to move them
over.

/Geoffrey.