gpython: Preimport golang and gevent from exactly the same place as std python would do

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.

gpython/__init__.py

@@ -44,10 +44,10 @@ from __future__ import print_function, absolute_import
 # pymain mimics `python ...`
 #
 # argv is what comes via `...` without first [0] for python.
-def pymain(argv):
-    import sys, code, runpy, six
+# init, if provided, is called after options are parsed, but before interpreter start.
+def pymain(argv, init=None):
+    import sys
     from os.path import dirname
-    from six.moves import input as raw_input
 
     run = None          # function to run according to -c/-m/file/interactive
     version = False     # set if `-V`
@@ -69,6 +69,7 @@ def pymain(argv):
             sys.argv = ['-c'] + argv # python leaves '-c' as argv[0]
             sys.path.insert(0, '')   # cwd
             def run():
+                import six
                 # exec with the same globals `python -c ...` does
                 g = {'__name__':    '__main__',
                      '__doc__':     None,
@@ -86,6 +87,7 @@ def pymain(argv):
             sys.argv = [mod] + argv
             sys.path.insert(0, '')  # cwd
             def run():
+                import runpy
                 # search sys.path for module and run corresponding .py file as script
                 runpy.run_module(mod, init_globals={'__doc__': None},
                                  run_name='__main__', alter_sys=True)
@@ -126,6 +128,8 @@ def pymain(argv):
         sys.path.insert(0, '')  # cwd
 
         def run():
+            import code
+            from six.moves import input as raw_input
             # like code.interact() but with overridden console.raw_input _and_
             # readline imported (code.interact mutually excludes those two).
             try:
@@ -146,6 +150,8 @@ def pymain(argv):
 
 
     # ---- options processed -> start the interpreter ----
+    if init is not None:
+        init()
 
     # handle -V/--version
     if version:
@@ -298,7 +304,12 @@ def main():
             raise RuntimeError('gpython: unknown -X option %s' % opt)
     argv = argv_ + argv
 
-    # initialize according to selected runtime
+    # init initializes according to selected runtime
+    # it is called after options are parsed and sys.path is setup correspondingly.
+    # this way golang and gevent are imported from exactly the same place as
+    # they would be in standard python after regular import (ex from golang/
+    # under cwd if run under `python -c ...` or interactive console.
+    def init():
         if gpy_runtime == 'gevent':
             # make gevent pre-available & stdlib patched
             import gevent
@@ -332,7 +343,7 @@ def main():
         sys.version += (' [GPython %s] [%s]' % (golang.__version__, gpy_verextra))
 
     # tail to pymain
-    pymain(argv)
+    pymain(argv, init)
 
 if __name__ == '__main__':
     main()