Updating the message when the user hits a conflict w/ installed packages

[pradyunsg]

Environment

• pip version: master today.
• Python version: all
• OS: all

Description

We've decided to update the error message presented to a user, when the final result set of packages will contain dependency conflicts.

Context: #8513, #7744 and a recent team meeting regarding the new resolver rollout.

Expected behavior

The behavior has to be changed in two ways, each related to a different part of the resolver rollout plan:

• The legacy resolver would present a message nudging users to realize that the upcoming new resolver might behave differently (see Implement step 1 in resolver rollout process #8513)
• The new resolver would present a message nudging users "hey, do you want the resolver to always be strict!? tell us!" (see Warn users about dependency conflicts when updating other packages #7744 (comment))

How to Reproduce / Output

To get the current message, run:

$ pip install six==1.10.0
$ pip install cherrypy==11.0.0
Collecting cherrypy==11.0.0
  Downloading CherryPy-11.0.0-py2.py3-none-any.whl (435 kB)
     |████████████████████████████████| 435 kB 5.3 MB/s 
Requirement already satisfied: six in /Users/pradyunsg/.virtualenvs/tmp-7af976d02abc202/lib/python3.8/site-packages (from cherrypy==11.0.0) (1.10.0)
Collecting portend>=2.1.1
  Downloading portend-2.6-py2.py3-none-any.whl (5.1 kB)
Collecting cheroot>=5.2.0
  Downloading cheroot-8.3.0-py2.py3-none-any.whl (86 kB)
     |████████████████████████████████| 86 kB 5.9 MB/s 
Collecting tempora>=1.8
  Downloading tempora-3.0.0-py2.py3-none-any.whl (14 kB)
Collecting more-itertools>=2.6
  Using cached more_itertools-8.4.0-py3-none-any.whl (43 kB)
Collecting jaraco.functools
  Downloading jaraco.functools-3.0.1-py3-none-any.whl (6.7 kB)
Collecting pytz
  Using cached pytz-2020.1-py2.py3-none-any.whl (510 kB)
ERROR: cheroot 8.3.0 has requirement six>=1.11.0, but you'll have six 1.10.0 which is incompatible.
Installing collected packages: more-itertools, jaraco.functools, pytz, tempora, portend, cheroot, cherrypy
Successfully installed cheroot-8.3.0 cherrypy-11.0.0 jaraco.functools-3.0.1 more-itertools-8.4.0 portend-2.6 pytz-2020.1 tempora-3.0.0

Note the line starting with "ERROR:".

[pradyunsg]

/cc @ei8fdb @nlhkabu for their inputs on the messages to present to the users in both these cases.

[nlhkabu]

Warn users the new behaviour might behave differently

Warning: Your project may soon experience errors when pip updates the way that it resolves dependency conflicts.
Use --use-feature=2020-resolver to test your project with the new resolver before it becomes the default.

Nudge users "hey, do you want the resolver to always be strict!? tell us!"

4 variations depending on whether the user is upgrading or installing, and depending on how many packages they are requesting:

Warning: Pip will install your package and it's dependencies without taking into account other packages you already have installed. This may cause an uncaught dependency conflict.
If you would like pip to take your other packages into account, please tell us here: issue_tracker_url

Warning: Pip will install your packages and their dependencies without taking into account other packages you already have installed. This may cause an uncaught dependency conflict.
If you would like pip to take your other packages into account, please tell us here: issue_tracker_url

Warning: Pip will upgrade your package and it's dependencies without taking into account other packages you already have installed. This may cause an uncaught dependency conflict.
If you would like pip to take your other packages into account, please tell us here: issue_tracker_url

Warning: Pip will upgrade your packages and their dependencies without taking into account other packages you already have installed. This may cause an uncaught dependency conflict.
If you would like pip to take your other packages into account, please tell us here: issue_tracker_url

[nlhkabu]

ping @pradyunsg - can you please confirm that these are accurate?
ping @ei8fdb - can you please review copy and make suggestions if you feel we can make it better.

Thanks both :)