Fix when pip is invoked as python /path/to/pip/

Because pip ships with an __main__.py file, one valid way to invoke it
is: python /usr/lib/python3.6/site-packages/pip/.  This results in
__package__ being the empty string   This, in turn, triggers the code to
add the pip wheel to sys.path.  Unfortunately, while this is fine to do
when it's restricted to a wheel just containing pip, it isn't okay to do
when it is site-packages as that will mean that other things in
site-packages could end up overriding things in the stdlib
(*cough*enum34*cough*).

Check file extension to limit when we end up adding the extra path to
sys.path.

src/pip/__main__.py

@@ -18,7 +18,10 @@
     # Resulting path is the name of the wheel itself
     # Add that to sys.path so we can import pip
     path = os.path.dirname(os.path.dirname(__file__))
-    sys.path.insert(0, path)
+    # __package__ could have also been None if pip was invoked with the path
+    # to the pip directory.  ie: as python /path/to/pip/
+    if path.endswith('.whl'):
+        sys.path.insert(0, path)
 
 from pip._internal.cli.main import main as _main  # isort:skip # noqa