What's the problem this feature will solve?

Tons of users are experiencing problems related to #5599, as evidenced by the traffic that we're getting to related issues (past 2 weeks):

This probably leads to a lot of negative feelings not just for pip, but the Python packaging ecosystem in general.

Describe the solution you'd like

Move the current entrypoint from pip._internal.main:main to pip._internal.cli.main:main (or some other location).

Create a function for every known past entrypoint:

• pip._internal.main:main
• pip._internal:main
• pip:main

At the very least the functions should invoke the correct current pip entrypoint. This alone would prevent a lot of needless suffering and should be easy to implement and test.

Later we could also consider printing a warning, but I'd rather handle that as a separate issue since this is useful on its own.

Alternative Solutions

• do nothing

Additional context

• pip install --user --upgrade #7493
• pip is not working #7403
• Help? pip won't work #7396
• Why I can't install pip #7369
• what problems ?Help me!! #7368
• I cant #7366
• TypeError: 'module' object is not callable #7264
• pip does not work after upgrade to v19.3 #7223
• Moving pip main function to own module broke dependent packages #7220
• 19.3 doesn't work after upgrading from 19.2.3 #7209
• Trying to execute module as a function #7206
• pip 19.3 breaking on CircleCI sys.exit(main()) TypeError: 'module' object is not callable #7205
• pip install --upgrade pip breaks pip (new version 19.2.3) #6927
• "cannot import name 'main'" on SO
• "TypeError: 'module' object is not callable" pip on SO
• more