Fix issue pytest_plugins as string was marking wrong modules for rewrite

[nicoddemus]

Unfortunately due to a small bug on how the pytest_plugins variable was handled, we are currently rewriting assertions in much more modules than we initially thought:

pytest_plugins = 'setuptools.tests.fixtures'

The code that handles that in config.py was expecting pytest_plugins to be a list[str] only:

    def consider_module(self, mod):
        plugins = getattr(mod, 'pytest_plugins', [])
        self.rewrite_hook.mark_rewrite(*plugins)
        self._import_plugin_specs(plugins)

So it would end up marking each letter in "setuptools.tests.fixtures" as an individual module, which basically means rewriting every module which starts with s, e, t, u and so on.

Coupling that with some advanced bootstrapping done in setuptools between each test and a lot of tests were failing quite catastrophically.

Initially I started the investigation for this issue by implementing the optional find_spec API, but it was only hiding the real bug so I decided to not touch that for now.

Beside testing locally, I opened pypa/setuptools#768 to make sure the fix worked. 😅

This fixes #1888

[coveralls]

Coverage increased (+0.06%) to 93.088% when pulling c8a366e on nicoddemus:find-spec-rewrite-hook into 82218e4 on pytest-dev:master.

[coveralls]

Coverage increased (+0.06%) to 93.088% when pulling edf8283 on nicoddemus:find-spec-rewrite-hook into 82218e4 on pytest-dev:master.

[The-Compiler]

That must've been a tricky one - thanks!