In 4312a4d3f4 MiniYaml merging was adjusted. One effect of this change was that duplicate keys in files that did not previously require merging was previously allowed, but was now an error. (Test case `TestMergeConflictsNoMerge`)
The installer files were relying on the previous behaviour to allow multiple `ContentPackage` keys. The above change caused a regression where attempting to manage mod content would crash due to now erroring on the duplicate keys.
We fix the issue by applying a unique ID suffix, as is a common pattern elsewhere in our yaml files, and teach InstallFromSourceLogic to recognise and strip it.
ContentPackages are defined in mod.yaml and list Installers that support them, but then the Installers and their SourceActions knew nothing about ContentPackages.
Also added BeforeInstall and AfterInstall sections for SourceActions in the Installers.
