Refactor task proxy

Remove task proxy class generation logic. Move `cylc.task_types.task` module to
`cylc.task_proxy` and rename the `task` class as `TaskProxy`. Each task proxy
is no longer its own class, but they all belong to the `TaskProxy` class.

Allow `-`, `+`, `%`, and `@` characters as task names.

Simplify logic in various places.

More pep8 and pylint fixes.