Disallow non-wheel build dependencies (PEP 518)

Merge pull request #4987 from pradyunsg/fix/fork-bombing

src/pip/_internal/wheel.py

@@ -624,7 +624,7 @@ class WheelBuilder(object):
         # Ignore the --no-binary option when installing the build system, so
         # we don't recurse trying to build a self-hosting build system.
         finder = copy.copy(self.finder)
-        finder.format_control = FormatControl(set(), set())
+        finder.format_control = FormatControl(set(), set([":all:"]))
         urls = [finder.find_requirement(InstallRequirement.from_line(r),
                                         upgrade=False).url
                 for r in reqs]