Bug#844725: libproj12 should conflict with libproj9

[Gianfranco Costamagna]

Source: proj
Version 4.9.3-1
Severity: Serious
Justification: breaks unrelated software when both libraries are installed.

Hi, as said, it took me two days to debug to understand this failure
https://objectstorage.prodstack4-5.canonical.com/v1/AUTH_77e2ada1e7a84929a74ba3b87153c0ac/autopkgtest-zesty/zesty/i386/m/mipp/20161118_053933_5e50e@/log.gz

the python binding fails and segfaults when both libproj9 and libproj12 are installed

(Reading database ... 65663 files and directories currently installed.)
Removing autopkgtest-satdep (0) ...
autopkgtest [05:39:15]: test python2: [-----------------------
=== python2.7 ===
test_tsx (test_xsar.Test) ... Segmentation fault (core dumped)
autopkgtest [05:39:23]: test python2: -----------------------]
autopkgtest [05:39:23]: test python2:  - - - - - - - - - - results - - - - - - - - - -
python2              FAIL non-zero exit status 139
autopkgtest [05:39:23]: @@@@@@@@@@@@@@@@@@@@ summary
python2              FAIL non-zero exit status 139

a no-change rebuild of mipp with both runtime libraries installed triggers the failure.

Since you can't force people from autoremoving the old libraries, I think this is an RC bug, and some sort
of conflict with previous version should be added to prevent such segfaults from happening.

thanks for understanding,

Gianfranco