tracer: for bin -> usr/bin symlink on debian systems we might to track both paths

[yarikoptic]

somewhat more info on VIDA-NYU/reprozip#390 but the overall issue is:

❯ ls -ld /bin
lrwxrwxrwx 1 root root 7 Nov  5  2019 /bin -> usr/bin/

❯ dpkg -S /{,usr/}bin/ls
coreutils: /bin/ls
dpkg-query: no path found matching pattern /usr/bin/ls

❯ dpkg -S /{,usr/}bin/whoami
dpkg-query: no path found matching pattern /bin/whoami
coreutils: /usr/bin/whoami

so for some binaries package would know about /usr/bin path and for other, /bin paths. In both cases users might run one or another and reprozip might dereference symlink (VIDA-NYU/reprozip#390) so we could end up with /usr/bin paths. I think we can "hard code" logic that iff issymlink('/bin'): trace both paths .