stylix: clean up `fromOs`

[danth]

A common question/issue is that the fallback default value for stylix.image is null, which is not a path or package. This produces a type error when it should simply state that the option has not been set.

This PR removes the fromOs function, and replaces it with a module which copies listed settings.

• The module is only loaded on the OS side, so it doesn't need to check that an OS configuration exists to copy from.
• The module injects extra Home Manager modules alongside the main one, so nothing is evaluated when using Home Manager standalone.
• All the options to be copied are listed in a central location.
• Two of the options are copied conditionally, to match previous behaviour.
• Copied values use mkDefault, again to match previous behaviour.

Fixes #406 Fixes #345

[danth]

Regarding my two comments above, the easiest solution would be to merge followSystem and autoImport into a single option, such as enableHomeManagerIntegration.

[danth]

A new use of fromOs was added in #372 (note to self)

[danth]

Another new usage in #244

[danth]

This now respects followSystem as before, however it only works when autoImport is also being used.

I doubt anyone was actually using followSystem without autoImport, given that it was only applicable to multi-user configurations, and even then was probably not the best way to organise things.

I think this is ready to merge :)