These are just bugfixes to already merged code:
* Fix nested bracket parsing in PEP508 marker parser.
* Fix the sys_platform constants, which I noticed in bazel-contrib#2629 but they got
  also pointed out in bazel-contrib#2766.
* Port some of python tests for requirement parsing and improve the
  implementation. Those tests will be removed in bazel-contrib#2629.
* Move the platform related code to a separate file.
* Rename `pep508_req.bzl` to `pep508_requirement.bzl` to follow the
  convention.

All of the bug fixes have added tests.

Work towards bazel-contrib#2423.