Respect PYTEST_DONT_REWRITE for plugins too.

[anntzer]

See #2995.

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[coveralls]

Coverage decreased (-0.0008%) to 92.748% when pulling c8e7d1a on anntzer:dont-rewrite-plugin into 5c6d773 on pytest-dev:master.

[RonnyPfannschmidt]

the python guide on sets proposes to write this as something more like

set(names).intersection(sys.modules).difference(self._rewritten_names) it would be nice to use that for the cleanup

[nicoddemus]

LGTM, thanks a lot @anntzer!

[nicoddemus]

(Note to maintainers: we have to remember to manually close the issue manually)

[coveralls]

Coverage decreased (-0.0008%) to 92.748% when pulling 45e7734 on anntzer:dont-rewrite-plugin into 5c6d773 on pytest-dev:master.

[coveralls]

Coverage increased (+0.006%) to 92.755% when pulling 45e7734 on anntzer:dont-rewrite-plugin into 5c6d773 on pytest-dev:master.