v2021 ← v2022dev2: gen_debhelper_files.pl: fix build on apertis

Apertis scripts build the package in a subfolder of the source tree, leading gen_debhelper_files.pl to find debian directories in the upper levels. As it uses only the first entry, but fails if more than one are found, this leads to a build failure.

This patch uses a less strict test to avoid this.