upstream check heuristic: detect subdirectory false-positives

[stapelberg]

Sometimes the heuristic results in a false-positive result: the package is shown as up-to-date, but a new major version is available in a different subdirectory. Some projects (e.g. util-linux) publish release within a major version in their own subdirectory, e.g. https://mirrors.edge.kernel.org/pub/linux/utils/util-linux/v2.32/util-linux-2.32.1.tar.xz.

The heuristic should be changed to detect that situation.