Callable main module patch: Support Python 2.7

Intentionally left out as a separate commit,
so it can be reverted once Python 2 support is dropped.

callable-main.patch

@@ -7,10 +7,10 @@ index 8c0e4c58..76e280e5 100755
  import pip._internal.utils.inject_securetransport  # noqa
 +from pip._internal import main  # noqa
 diff --git a/src/pip/_internal/main.py b/src/pip/_internal/main.py
-index 1e922402..e52e04d8 100644
+index 1e922402..d3df58b3 100644
 --- a/src/pip/_internal/main.py
 +++ b/src/pip/_internal/main.py
-@@ -45,3 +45,15 @@ def main(args=None):
+@@ -45,3 +45,30 @@ def main(args=None):
      command = create_command(cmd_name, isolated=("--isolated" in cmd_args))
  
      return command.main(cmd_args)
@@ -20,9 +20,24 @@ index 1e922402..e52e04d8 100644
 +# This is needed to be able to use this pip in ensurepip of older Pythons
 +# without patching all the Pythons.
 +
-+class _CallableModule(sys.modules[__name__].__class__):
++# In Python 3.5+, we can just inherit, define __call__ and override
++# sys.modules[__name__].__class__, however, that is not possible in 2.7.
++
++class _CallableModule(type(sys.modules[__name__])):
++    def __init__(self):
++        super(_CallableModule, self).__init__(__name__)
++        self._main = sys.modules[__name__]
++        sys.modules[__name__] = self
++        self.__doc__ = self._main.__doc__
++
 +    def __call__(self, *args, **kwargs):
 +        return main(*args, **kwargs)
 +
++    def __dir__(self):
++        return dir(self._main)
 +
-+sys.modules[__name__].__class__ = _CallableModule
++    def __getattr__(self, attr):
++        return getattr(self._main, attr)
++
++
++_CallableModule()