This redoes 0abf873d (Install egg from wheel only if explictly required),
because that switch to install wheels only when explicitly requested
broke things in several places[1,2]. We can avoid such breakage if
instead of rejecting all wheels by default, we accept wheels, but allow
only pure-python wheels to be installed.

As slapos!1026 (comment 138639)
shows detecting whether a dist is pure-python wheel is just

	(dist.precedence == WHL_DIST and dist.platform is None)

So let's use this and rework egg installer accordingly.

This patch is completely untested. However the way to check whether a
wheel is source-only was tested at slapos!1026 (comment 138639).

If this patch is accepted and new slapos.buildout release is made, then
all :whl usage in slapos.git can be dropped.

/cc @tomo, @jerome, @jm

[1] slapos!1026
[2] slapos!1026 (comment 138590)