During a build of extension module within `pip wheel` the source directory is
recursively copied in a temporary directory.

See https://github.com/pypa/pip/issues/7555

When the temporary directory is inside the source directory
(for example by setting `TMPDIR=$PWD/tmp`) this caused an infinite recursion
that ended in:

    [Errno 36] File name too long

We prevent that buy never copying the target to the target in _copy_source_tree.

Fixes https://github.com/pypa/pip/issues/7872