Auto merge of #6889 - Y-Nak:refactor-unit-types, r=flip1995

Refactor unit types

Ref:  #6724
r? `@flip1995`

Changes:
1. Extract `unit_types` from `types` group.
2. Move lints of `unit_types` to their own modules.

Notes:
Other lints of `unit_types` is still scattered around the `clippy_lints`, e.g. `result_unit_err` or `option_map_unit_fn`.
These should be addressed in another PR.

changelog: none