fix(pypi): correctly override default indexes as documented in our docs

[aignas]

This fixes how defaults for pip.default(index_url) is interpreted if the
--index-url is used in a requirements.txt file arguments. From now on the
default set in pip.default takes precedence if it is set to something else
than https://pypi.org/simple.

Now behaviour is as such:

• default - https://pypi.org/simple is used or if there is a
  requirements.txt value, it will take precedence.
• user specifies https://pypi-internal.mirror -
  https://pypi-internal.mirror is used at all times.
• user specifies https://pypi.org/simple - same as default

Closes #4123