gpython: Move sys.executable setup from main to pymain

This is generic step and should be done under both gpython and standard
python. Handling sys.executable inside pymain offloads users from
setting up sys.executable by themselves, e.g. as in here:

https://lab.nexedi.com/nexedi/slapos/blob/46ed2afe/component/pygolang/buildout.cfg#L39-43

gpython/__init__.py

@@ -50,9 +50,46 @@ _pyopt_long = ('version',)
 # init, if provided, is called after options are parsed, but before interpreter start.
 def pymain(argv, init=None):
     import sys
+
+    # sys.executable
+    # on windows there are
+    #   gpython-script.py
+    #   gpython.exe
+    #   gpython.manifest
+    # and argv[0] is gpython-script.py
+    exe  = argv[0]
     argv = argv[1:]
+    if exe.endswith('-script.py'):
+        exe = exe[:-len('-script.py')]
+        exe = exe + '.exe'
+    sys._gpy_underlying_executable = sys.executable
+    sys.executable  = exe
+
+    import os
     from os.path import dirname, realpath
 
+    # `python /path/to/gpython` adds /path/to to sys.path[0] - remove it.
+    # `gpython file` will add path-to-file to sys.path[0] by itself, and
+    # /path/to/gpython is unneccessary and would create difference in behaviour
+    # in between gpython and python.
+    exedir = exe[:exe.rindex(os.sep)] # dirname, but os.path cannot be imported yet
+    if sys.path[0] == exedir:
+        del sys.path[0]
+    else:
+        # buildout injects `sys.path[0:0] = eggs` into python scripts.
+        # detect that and remove sys.path entry corresponding to exedir.
+        if not _is_buildout_script(exe):
+            raise RuntimeError('pymain: internal error: sys.path[0] was not set by underlying python to dirname(exe):'
+                    '\n\n\texe:\t%s\n\tsys.path[0]:\t%s' % (exe, sys.path[0]))
+        else:
+            if exedir in sys.path:
+                sys.path.remove(exedir)
+            else:
+                raise RuntimeError('pymain: internal error: sys.path does not contain dirname(exe):'
+                    '\n\n\texe:\t%s\n\tsys.path:\t%s' % (exe, sys.path))
+
+
+
     run = None          # function to run according to -c/-m/file/interactive
     version = False     # set if `-V`
     warnoptions = []    # collected `-W arg`
@@ -252,19 +289,6 @@ def main():
         sys.setdefaultencoding('utf-8')
         delattr(sys, 'setdefaultencoding')
 
-    # sys.executable
-    # on windows there are
-    #   gpython-script.py
-    #   gpython.exe
-    #   gpython.manifest
-    # and argv[0] is gpython-script.py
-    exe  = sys.argv[0]
-    argv = sys.argv[1:]
-    if exe.endswith('-script.py'):
-        exe = exe[:-len('-script.py')]
-        exe = exe + '.exe'
-    sys._gpy_underlying_executable = sys.executable
-    sys.executable  = exe
 
     # import os to get access to environment.
     # it is practically ok to import os before gevent, because os is always
@@ -272,32 +296,12 @@ def main():
     # no harm wrt gevent monkey-patching even if we import os first.
     import os
 
-    # `python /path/to/gpython` adds /path/to to sys.path[0] - remove it.
-    # `gpython file` will add path-to-file to sys.path[0] by itself, and
-    # /path/to/gpython is unneccessary and would create difference in behaviour
-    # in between gpython and python.
-    exedir = exe[:exe.rindex(os.sep)] # dirname, but os.path cannot be imported yet
-    if sys.path[0] == exedir:
-        del sys.path[0]
-    else:
-        # buildout injects `sys.path[0:0] = eggs` into python scripts.
-        # detect that and remove sys.path entry corresponding to exedir.
-        if not _is_buildout_script(exe):
-            raise RuntimeError('gpython: internal error: sys.path[0] was not set by underlying python to dirname(gpython):'
-                    '\n\n\tgpython:\t%s\n\tsys.path[0]:\t%s' % (exe, sys.path[0]))
-        else:
-            if exedir in sys.path:
-                sys.path.remove(exedir)
-            else:
-                raise RuntimeError('gpython: internal error: sys.path does not contain dirname(gpython):'
-                    '\n\n\tgpython:\t%s\n\tsys.path:\t%s' % (exe, sys.path))
-
     # extract and process `-X gpython.*`
     # -X gpython.runtime=(gevent|threads)    + $GPYTHON_RUNTIME
     sys._xoptions = getattr(sys, '_xoptions', {})
     argv_ = []
     gpy_runtime = os.getenv('GPYTHON_RUNTIME', 'gevent')
-    igetopt = _IGetOpt(argv, _pyopt, _pyopt_long)
+    igetopt = _IGetOpt(sys.argv[1:], _pyopt, _pyopt_long)
     for (opt, arg) in igetopt:
         if opt == '-X':
             if arg.startswith('gpython.'):
@@ -318,7 +322,7 @@ def main():
         if opt in ('-c', '-m'):
             break
 
-    argv = [exe] + argv_ + igetopt.argv
+    argv = [sys.argv[0]] + argv_ + igetopt.argv
 
     # init initializes according to selected runtime
     # it is called after options are parsed and sys.path is setup correspondingly.