Debian distro not correctly detected with this os-release

This /usr/lib/os-release
```
PRETTY_NAME="Debian GNU/Linux 10 (buster)"
NAME="Debian GNU/Linux"
VERSION_ID="10"
VERSION="10 (buster)"
VERSION_CODENAME=buster
ID=debian
HOME_URL="https://www.debian.org/"
SUPPORT_URL="https://www.debian.org/support"
BUG_REPORT_URL="https://bugs.debian.org/"
```
was not detected correctly in rootfs pipeline.

1. we should ensure that we can get to this path too
2. we should be more flexible with the paths (and leading segments)
3. in all cases, checking for system-installed packages should not be dependent on the detected distro. We should try Debian, RPM, etc. and all supported types in all FS and/or layers analyzed in a scancode.io pipeline (which impacts the code here, in scancode-toolkit and in scancode.io)