Improve tracebacks for ImportErrors in conftest

[nicoddemus]

Here's an conftest import error on master:

λ pytest 
Traceback (most recent call last):
  File "c:\pytest\src\_pytest\config\__init__.py", line 381, in _getconftestmodules
    return self._path2confmods[path]
KeyError: local('c:\\pytest\\.tmp\\conftest-err\\tests')

During handling of the above exception, another exception occurred:
Traceback (most recent call last):
  File "c:\pytest\src\_pytest\config\__init__.py", line 412, in _importconftest
    return self._conftestpath2mod[conftestpath]
KeyError: local('c:\\pytest\\.tmp\\conftest-err\\tests\\conftest.py')

During handling of the above exception, another exception occurred:
Traceback (most recent call last):
  File "c:\pytest\src\_pytest\config\__init__.py", line 418, in _importconftest
    mod = conftestpath.pyimport()
  File "c:\pytest\.env36\lib\site-packages\py\_path\local.py", line 668, in pyimport
    __import__(modname)
  File "c:\pytest\src\_pytest\assertion\rewrite.py", line 290, in load_module
    six.exec_(co, mod.__dict__)
  File "c:\pytest\.tmp\conftest-err\tests\conftest.py", line 10, in <module>
    some_import()
  File "c:\pytest\.tmp\conftest-err\tests\conftest.py", line 6, in some_import
    other_import()
  File "c:\pytest\.tmp\conftest-err\tests\conftest.py", line 2, in other_import
    import invalid_internal
ModuleNotFoundError: No module named 'invalid_internal'
ERROR: could not load c:\pytest\.tmp\conftest-err\tests\conftest.py

and here's the same error on this branch:

λ pytest .tmp\conftest-err\
ImportError while loading conftest 'c:\pytest\.tmp\conftest-err\tests\conftest.py'.
tests\conftest.py:10: in <module>
    some_import()
tests\conftest.py:6: in some_import
    other_import()
tests\conftest.py:2: in other_import
    import invalid_internal
E   ModuleNotFoundError: No module named 'invalid_internal'

---

Fix #3332

cc @The-Compiler

Thanks @feuillemorte for the initial take on this in #3345 👍

[blueyed]

<> would be a valid filename, no? Similar to foo><bar - so maybe use startswith/endswith at least?

[blueyed]

I see that it is just moved code though.

[nicoddemus]

Makes sense, but I'm not sure I would like to touch that now, there might be some corner case which is covered by that, plus <> are not valid characters for filenames AFAIK so I think we're safe. 😁

[coveralls]

Coverage increased (+0.04%) to 93.793% when pulling ef97121 on nicoddemus:traceback-import-error-3332 into 933de16 on pytest-dev:features.