x11/plasma6-plasma-desktop: Fix installation with XLibre. #578

This is a follow up of PR 291602.

This issue was known before XLibre was added to the ports tree, and we created the above PR, right after the XLibre merger to resolve it. Unfortunately the resulting changes didn't address the core issue[^1], so I created this GitHub PR as a follow-up.

In short, the problem is a runtime dependency conflict between x11/kde and x11-servers/xlibre-server, and it plays out like this:

• 
  x11/kde: Depends in runtime on x11/plasma6-plasma-desktop
  


• 
  x11/plasma6-plasma-desktop: $${\color{red} Depends\space in\space runtime\space on\space}$$ x11-drivers/xf86-input-libinput
  


• 
  x11-drivers/xf86-input-libinput: Depends in runtime on x11-servers/xorg-server
  


• 
  x11-servers/xorg-server: Totally conflicts with x11-servers/xlibre-server
  

Hence x11/kde conflicts with x11-servers/xlibre-server in runtime.

The conflict is reproducible by first installing x11-servers/xlibre-server with pkg on a test system/jail and then trying to install x11/kde.
After pkg fetches all the required dependencies and tries to install x11-servers/xorg-server, it prompts the user to remove the entirety of their XLibre installation.

To fix this issue, initially an admittedly more invasive method of FLAVORizing the KDE ports was suggested, but that's NOT what's being suggested here. Shortly after, we rethought the approach and suggested this current solution: that is to simply remove the runtime dependency of x11/plasma6-plasma-desktop on x11-drivers/xf86-input-libinput which is the key link in this conflict.
KDE plasma-desktop requires a header file from x11-drivers/xf86-input-libinput during build time only. There is nothing in KDE that would require it to be present in runtime. So it would be best to make plasma-desktop depend on xf86-input-libinput at build time only as you can see in the patch.

I also had a look at several Linux distributions, and they all do the same:

• Arch Linux: Only a makedepend.
  


• Void Linux: Only a makedepend.
  


• Debian: Only a builddepend.
  


• Alpine: Only a makedepend.
  

In the past 8 months, there have been quite a few cases of people trying to install KDE+XLibre, and facing this exact issue. Some examples:

Graham Perrin on r/FreeBSD
User on FBSD forums
Another user on the FBSD forums
Blog detailing Installation of FBSD on a 2012 Macbook

Given the circumstances, I'd like to kindly ask you to merge the PR. If possible, a back-port to Q3 would be appreciated so that the solution can be made available to users sooner.

CC @alonsobsd @DtxdF

[^1]: After PR 291602 was closed, commit 11923b7 was made referencing it. The changes unfortunately continued to impose a runtime dependency on x11-drivers/xf86-input-libinput in the resulting package, and our follow-up remained unanswered.