Formatting changes for dependencies

[mazunki]

Requested change for #2325.

This purely introduces consistency changes and formatting.

[MagnusS]

Thanks! Why is it better to use let..in here?

[mazunki]

It's not necessarily better per se, but variables such as ${out} and friends don't exist until after the derivation is created. This will be useful when we start exposing include paths and so forth.

I think it will also allow us to look at source files after patches are applied, but I haven't tested this yet.

[mazunki]

After these formatting changes, I will be able to use said ${out} and similar variables which don't exist before the derivation is complete the grant easy attribute access to include, lib and source paths of the dependencies for usage in other scripts.