move dummy ranges and clean imports in std.algorithm

1. Dummy ranges was moved into `std/internal/test/dummyrange.d.`
2. Imports was cleaned in `std.algorithm`
2. Imports was fixed in `std.range.*`
That's a documented unit test. The import is there so that the unittest is "self sufficient" no matter where it used. It wasn't an accident.

Please keep documentation in mind when removing imports (any import) from documented unit tests.