check_whence.py: ban link-to-a-link

check_whence.py: ban link-to-a-link

Using link-to-a-link reduces legibility and changes to the root link,
also changes the leafs. Where the latter may be undesired and in some
cases just wrong.

We have a couple of instances in-tree, so fix them up and ban the
combination.

One particularly good example, why we'd want this is:
https://gitlab.com/kernel-firmware/linux-firmware/-/merge_requests/272

In there we have the following:

    File: ti/tas2781/TAS2XXX1EB30.bin
    [snip]
    Link: TAS2XXX1EB3.bin -> ti/tas2781/TAS2XXX1EB3.bin
    Link: ti/tas2781/TAS2XXX1EB3.bin -> TAS2XXX1EB30.bin
    Link: TAS2XXX1EB30.bin -> ti/tas2781/TAS2XXX1EB30.bin

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>