Reimplement py_pytest_test macro as a rule.

[UebelAndre]

Currently the macro requires additional targets be generated under the hood so that inputs can be accurately processed before passing them to the pytest process. The ideal solution is to not rely on the py_test rule but to instead convert py_pytest_test into it's own rule so we can more efficiently do plumbing and more gracefully handle situations like transitions

bazelbuild/rules_python#1647 tracks the py_common feature request upstream which could provide a way to generate python executables in custom rules.