ini files looked in a directory when --pyargs is used

Pytest (3.2.2) looks for configuration files on directories named on command line, even if --pyargs option is given to interpret them as Python package names.

Reproduction:

$ cd /tmp
$ git clone https://github.com/pytest-dev/pytest pytest
$ python3 -mpytest --pyargs pytest
===================================== test session starts =====================================
platform linux -- Python 3.6.2, pytest-3.2.2, py-1.4.34, pluggy-0.4.0
rootdir: /tmp/pytest, inifile: tox.ini
plugins: xdist-1.20.0, forked-0.2, timeout-1.0.0, faulthandler-1.3.0, cov-2.4.0, hypothesis-3.12.0
collected 0 items                                                                              

================================ no tests ran in 0.00 seconds =================================

It correctly collects 0 tests since an installed pytest package contains no tests. However, the rootdir above looks wrong to me, I would expect it to be /tmp. When --pyarg is given on the command line, the items listed should be considered as package names, not directory names.

Start by reproducing python3 -mpytest --pyargs pytest from /tmp, then trace how command-line arguments are interpreted and how the root directory and ini file are selected. Done means package names passed with --pyargs are not treated as directories, the rootdir is /tmp, and a regression test covers the reported behavior.