... and everything that whole pygolang depends on in pygolang[all].

The reason we do this: currently golang.perf.benchlib needs NumPy, but
it would be not ok to depend whole pygolang on numpy. Similarly pybench
needs pytest, but it would be not ok to require pytest unconditionally.

Thus allow users to specify what they need from pygolang via extras,
like pygolang[x.perf.benchlib] or pygolang[cmd.pybench], with
pygolang[all] serving as merge point for all subpackages dependencies.

TODO in the future we'll, hopefully, also provide accumulation points,
like pygolang[x.perf] which contains everything pygolang[x.perf.*]
depends on.

Note: core functionality and gpython are provided always unconditionally.