Allow custom scheduler names in `--dist` command line argument

[amezin]

There seems to be no way to fix the argument validation, at least without complicating things unnecessarily.

So simply remove it. Instead, check pytest_xdist_make_scheduler return value.

Also, convert every built-in scheduler to a separate plugin.

Fixes: https://github.com/pytest-dev/pytest-xdist/issues/970