Moving pip main function to own module broke dependent packages

[hgibsonqb]

Environment

• pip version: 19.3
• Python version: 3.6.8
• OS: 4.9.184-linuxkit

We're managing dependencies with pipenv, version 2018.10.13

Description

As part of the 19.3 release, pip main function was moved to it's own module 09fd200

This is breaking packages that import and run pip.main with this error

 File "<string>", line 1, in <module>
TypeError: 'module' object is not callable

This change was not listed in the deprecation section for 19.3 https://pip.pypa.io/en/stable/news/#id1

Expected behavior

Expected dependent package to import and run pip main.

[pfmoore]

Pip's main function is notr a supported interface - see the documentation. So there is no need for a deprecation period if we change internal APIs, and anyone using those APIs must be prepared to deal with such changes.

[chrahunt]

As mentioned in #7204, a supported way to execute pip commands is

subprocess.check_call([sys.executable, '-m', 'pip', 'install', 'my_package'])

I will close this as a duplicate of the above.