find-debuginfo: Allow for specifying output dir for all *.list files

[Michal Domonkos]

This patch series adds a new switch to the find-debuginfo script that allows
for overriding the output directory which otherwise defaults to the builddir
argument. A test is included as well. See the commit messages for details.

The primary motivation is for RPM to avoid polluting the unpacked source tree
in --build-in-place mode, see the following discussion for details:

    https://github.com/rpm-software-management/rpm/discussions/3644