Implementing equery hasuse

The command equery hasuse <useflag> from app-portage/gentoolkit finds all packages that contain the given useflag.

In pms-utils, this would look like:

>>> def hasuse(useflag: pms_utils.atom.Useflag) -> list[pms_utils.repo.Package]:
...     ret = []
...     repo = pms_utils.repo.Repository("/var/db/repos/gentoo")
...     def check_package(package: pms_utils.repo.Package) -> bool:
...         for ebuild in package:
...             for iuse_elem in ebuild.metadata.IUSE:
...                 if iuse_elem.useflag == useflag:
...                     return True
...         return False
...     for category in repo:
...         for package in category:
...             if check_package(package):
...                 ret.append(package)
...     return ret