Change install paths, to make package usrmerge-compliant

Philip Handsrequested to merge
philh/debian-edu-install:usrmerge into master

This gets rid of the lintian warnings about installing things into /lib.

BTW I considered changing the references to the files, found in debian-edu-profile and e.g. 90debian-edu-mountoptions, but it strikes me as a waste of effort (and of a a little space too) so I decided against doing that. Policy states that one can assume that the symlinks exist, so no need to change those IMO (unless not doing so causes a problem at some point).

I also tried using branch2repo in order to test the result of using this, but the way branch2repo currently builds the mini-ISO means that it behaves as a standard mini-ISO rather than a thing that will install Debian Edu. I'm uncertain what's needed to fix that, but if you think it might be useful I'm happy to do so with pointers on how one can tell d-i to build a Debian Edu flavour mini-ISO.

You can see what I'm on about here: https://salsa.debian.org/philh/debian-edu-install/-/pipelines/970021

which if you click to expand the child pipelines to the right eventually leads you here: https://openqa.debian.net/tests/461088

which is a test using the newly built mini-ISO from the pipeline, attempting a Debian Edu install, which fails -- This is because this mini-ISO is prompting for things in the standard order, rather than the order that a proper Debian Edu medium should do things. So in this case when the test is hoping to see the Debian Edu Profile Selection screen, it's instead being prompted for the root password.

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