Explicitly inform user we can’t allow certain packages to be pinned

[uranusjr]

Via #2364. So it is probably not possible (without extensive project restructuring) to allow the user to install cetain packages. We should then tell the user that, instead of silently ignore the requirement.

A few points to discuss:

• When should the message be triggered? IMO it should be shown when certain packages are pinned by someone (either top-level or not) to a version that does not satisfy the current installation.
  • 5.0 installed, >=4.3 specified -> No message
  • 5.0 installed, ~=4.0 specified -> message
• What level should the message be? I think a warning would be enough (XXX dropped since it is required by Pipenv’s resolver), but a case can be made for an error (and locking failure).