Improve 'module not found' message

In #1567 it was men, user has spent a long time trying to figure out why their namespace package was not being found, reason being that pytest does not support namespace packages via --pyargs (#478).

One easy suggestion is to improve the error message, from:

ERROR: module or package not found: module.misc (missing __init__.py?)

To something like this:

ERROR: pytest cannot import module or package 'module.misc'
Possible reasons:
- Missing an __init__.py file
- Namespace package (not supported)
- Not reachable via PYTHONPATH
See <link to docs> for more information.

Bonus points if we link to a documentation where we have an explanation for each bullet point above.

_Originally posted by @nicoddemus in https://github.com/pytest-dev/pytest/issues/1567#issuecomment-1342649479_

[dimpase]

Our (for SageMath) main use of pytest would be to run doctests (in Python and in Cython). This can be done on per-file basis, in parallel (otherwise it takes many hours of wallclock time). But pytest makes it unnecessary hard to do things like pytest foo/*/* – all these missing __init__.py errors. Currently we have a homemade tool which uses a bit of Python doctest module for this, but it’s old and tired, and we really want an off the shelf solution instead.

[RonnyPfannschmidt]

No promises