This test was not particularly slow compared to the rest of the module.

Update the generated versioneer fixes to the current git, in order
to pull a bugfix that fixes the support for calling `build_ext` prior
to `build_py`.  This is part of Gentoo's workflow, and the current
version of generated files is requiring us to use workarounds.

This adds the missing `update_mapping`s step during cloning, as well as a new
`Feature` cloning step that prevents issues when features are copied to their
clones.

Since it is no longer necessary to clone `Constant`s, and they add extra work
for the merge rewrites, `clone_get_equiv`'s `copy_orphans` option has been
prevented from cloning `Constant`s.  The option is mostly applicable to
constants, but it has been retained for other non-`Constant` cases (just in
case) and backward compatibility.