pytester.Testdir.plugins is confusing and clunky

[goodboy]

pytester.TestDir.plugins is used two different ways:

• as a list of Python objects or str which are handed to the config parser
• as a list of strings handed as names to a subprocess

I propose just getting rid of it and instead having runpytest_subprocess and parseconfig accept it as a keyword arg like inline_run does.

It's also awkward that you have to manually mutate the list in order to use it.

[nicoddemus]

I'm fine with it: never used and in pytest's code base it is only used by test_resultlog.py in 3 places.