Improve assert mod not in mods error message

[lesteve]

Related to #9765, add a better error message than assert mod not in mods. This would have been super useful to figure out from the error message the difference from conftestpath and mod.__file__. Hopefully the #11708 will be merged and this error is less likely to occur, but at least if a variation of the issue occurs again there will be a better error message.

Here is the error on my reproducer https://github.com/lesteve/pytest-issue-9765-reproducer:

AssertionError: str(conftestpath)='C:\\msys64\\home\\lesteve\\dev\\pytest-issue-9765-reproducer\\project\\conftest.py'
has already been loaded as the module mod=<module 'project.conftest' from 'c:\\msys64\\home\\lesteve\\dev\\pytest-issue
-9765-reproducer\\project\\conftest.py'>

For now I keep an AssertionError but let me know if you think another kind of error is more appropriate.

It does not seem that easy to add a test for this ...

Let me know if you think this is worth adding a changelog for this or not!

Depending on how #11708 goes, this may need to be tweaked since a discrepancy between mod.__file__ and str(contestpath) is likely not how this problem will show up. It could even lead to red-herring, since it will be potentially fine that mod.__file__ and str(conftestpath) are not the consistent and people may be misled by reading comments like #9765 (comment) ...

[nicoddemus]

Indeed having a better error message is something we should have for all internal assert calls, thanks!

[bluetech]

Thanks, this would have been useful with the issue.

[bluetech]

I do think it will be useful to include the __file__ as a debugging aid - it may no longer be at fault but it's useful to see where mod came from.

[bluetech]

Made this update