Refactor handling of non-top-level pytest_plugins handling
Refactor handling of non-top-level pytest_plugins handling

Decided to move the 'if' logic together with the error message, as this leaves
the _importconftest method cleaner.

markshao/pytest,Akasurde/pytest,nicoddemus/pytest,The-Compiler/pytest,pytest-dev/pytest,nicoddemus/pytest,RonnyPfannschmidt/pytest,The-Compiler/pytest