Add content/pkgmgr unit tests

[rubenwardy]

This issue documents everything that should be tested with the content/pkgmgr. This issue can be closed when unit tests are added

Installing content

• Updating
• Installing but already exists (return error)
• Finding base folder

Enabling/disabling mods

• Dependency cycle
• Enabling modpack which depends on external mods (both modpack and component mods)
• Enabling mod with multiple candidates, one of which pre-enabled
• Enabling mod with unsatisfied dep
• Enabling mod which depends on world/game mod
• Recursively enabling dependencies

Mod Configuration / Resolve Dependencies

• Dependency cycles
• Missing hard dependency
• Mod A opt depends on mob B with missing mod C
• Mod that depends on world/game mod
• Multiple mods with same name, with no set virtual path load_mod_x = true
• Game mod opt-dep breaking

Content discovery

• Finds content from user dir and share
• Game mods
• Nested modpacks
• Virtual paths
• Content type detection
• Modname validation
• Game name/titles?