Add pip.main as func raising clean error

src/pip/__init__.py

@@ -1 +1,17 @@
 __version__ = "10.0.0"
+
+
+def main(*args, **kwargs):
+    """
+    This is an importable main func.
+    Its only purpose is to raise a clean error for users who call `pip.main()`
+    -- a usage which was never supported.
+    """
+    raise RuntimeError(
+        "pip.main() is unsupported and should not be used. "
+        "If you want to invoke pip from within your program, see the "
+        "documentation here: "
+        "https://pip.pypa.io/en/latest/user_guide/#using-pip-from-your-program"
+        "\n"
+        "You may find that pip.main() works on pip<10, but pip cannot and "
+        "does not support or condone such usage.")