Write the self-update packages to /.packages.self_update (bsc#1175614)

[lslezak]

The Problem

• See https://bugzilla.suse.com/show_bug.cgi?id=1175614
• The inst-sys contains the list of included packages (and their versions) in the /.packages.* files
• However, after YaST applies the self-update packages that is not true anymore
• Additionally it would help with debugging some problems if we could quickly get the list of updated packages in the inst-sys (could be used for debugging bug https://bugzilla.suse.com/show_bug.cgi?id=1175374)

The Solution

• Write the list of the updated packages to the /.packages.self_update file using the same format as the installation-images use for the other /.packages.* files.

Testing

• Added a unit test
• Manually tested in the SP2 installer, after running the self-update step the /.packages.self_update file contained these lines:

libsolv-tools [0.7.14-3.5.1.x86_64]
libstorage-ng-ruby [4.2.76-3.3.2.x86_64]
libstorage-ng1 [4.2.76-3.3.2.x86_64]
libzypp [17.24.1-3.11.1.x86_64]
release-notes-sles [15.2.20200729-3.3.1.noarch]
skelcd-control-leanos [15.2.12-3.3.1.x86_64]
yast2-installation [4.2.43-3.3.1.noarch]
yast2-packager [4.2.64-3.3.4.x86_64]
yast2-pkg-bindings [4.2.8-3.3.4.x86_64]
yast2-storage-ng [4.2.111-3.4.2.x86_64]

[yast-bot]

✔️ Public Jenkins job #122 successfully finished
✔️ Created OBS submit request #831235

[yast-bot]

✔️ Internal Jenkins job #49 successfully finished
✔️ Created IBS submit request #225622

[coveralls]

Coverage increased (+0.03%) to 29.888% when pulling d7aaf34 on packages.self_update into 593287b on master.