Fix .deepOverride on poppler, glib by moving the default derivation into callPackage call

[MarcWeber]

No description provided.

[MarcWeber]

This patch will be important for another patch updating cups which is not ready yet

[vcunat]

Looks good to me, no hashes change, etc. Pushed.

I'm a bit unsure about the overriding methods and how to behave not to break them. Are these documented somewhere?

[MarcWeber]

vcunat: In the current case there was nothing to break because using .override dropped the default derivation in those cases making it impossible to use at all. Those two looked like this: default // { inherit way1; inherit way 2}; When using override the default // thing was gone.
Documentation? Mmh Overriding is very very very complicated - I'm not sure I understand all details myself. Its because __override and similar things play together. In short it means when compiling firefox you can replace a dependency in all prerequisites. Using applyGlobalOverrides is an alternative. See #1120 to learn about the current use case (force using the same ghostscript for multiple packages).

[vcunat]

Ah, I think I see now. So the key is that the package attributes are unmodified results of callPackage, then they are overridable.

But on the whole, it would be good to compose some docs on overriding, e.g. on the wiki. If someone who understands it did it, that would be great (if someone like that even exists :)