pygolang: Provide python-interpreter:exe

During my recent work I needed to use path for generated interpreter in
several places and found the need to repeat
${buildout:bin-directory}/${python-interpreter:interpreter} both tiring
and error-prone, because the knowledge where executable is placed
is implicitly used and relied upon.

On the other hand:

- pygolang already provides ${gpython:exe} as reference to the place
  where gpython is installed (see e1d269b4)

- pygolang already uses :exe for interpreter generated to accompany
  pyprog (see 0ee52376 and e328aa49)

So python-interpreter not providing :exe is an oversight and the logical
fix is to start providing python-interpreter:exe as well.

-> Do it and convert */software.cfg throughout the tree, where
python-interpreter is found, to use it.

/cc @jerome, @Tyagov, @alain.takoudjou, @xavier_thompson, @levin.zimmermann
/reviewed-on !1456