Customize package removal

[kgilmer]

Summary

This PR extracts the list of packages to prune from filesystem.manifest-desktop. This would allow users to remove packages required during the installation process but not desired on the target system.

Testing Done

I generated an ISO and checked the files after the image was created to verify the expected file contents:

$ diff filesystem.manifest filesystem.manifest-desktop 
60d59
< casper 1.445.1
115,116d113
< discover 2.1.2-8
< discover-data 2.2013.01.11ubuntu1
453d449
< laptop-detect 0.16
584d579
< libdiscover2 2.1.2-8
1250d1244
< lupin-casper 0.57build1
1295d1288
< os-prober 1.74ubuntu2
1505,1509d1497
< ubiquity 20.04.15.11
< ubiquity-casper 1.445.1
< ubiquity-frontend-gtk 20.04.15.11
< ubiquity-slideshow-ubuntu 160
< ubiquity-ubuntu-artwork 20.04.15.11