GPython preimports golang and gevent, but until now, it was preimporting
them before adjusting sys.path according to given -c/-m/file/... And
this was leading to difference in behaviour with standard Python where
e.g. `import golang` might result in different files loaded if e.g.
golang/ is there on cwd and -c/-m was given.

-> Rework gpython startup so that golang/gevent preimport happens after
sys.path initialization.

This fixes `tox -e py27-gevent` which was previously failing on
test_defer_excchain_dump_pytest / test_defer_excchain_dump_ipython
because dumped paths were different than expected ones.

It, however, breaks test_defer_excchain_traceback on py27-gevent.  ->
We'll do more fixups to sys.path handling in the next patch which will
fix that failure as well.