Deprecate "pytest_funcarg__" prefix for fixtures

[nicoddemus]

Start to issue deprecation warnings when fixtures are declared using this legacy idiom, to be removed at some point later in time.

[nicoddemus]

cc @obestwalter

[nicoddemus]

Not sure if it's worth, as keeping support for this old form is simple enough too

[RonnyPfannschmidt]

i want to see this removed :) it complicates things as its not sanely layered

[obestwalter]

It's not explicitly deprecated yet. Shall we add a deprecation warning before 3.0? I just grepped through the code and there are loads of tests using it. Updating those tests to use the fixture decorator would be a nice low hanging fruit task.

[nicoddemus]

OK, I think we should introduce a deprecation warning for 3.0 then. Added it to the Wiki.

[RonnyPfannschmidt]

done in #1714