refactor(filesource): replace repetitive #[cfg()] usages with cfg_if!{}

[rami3l]

It occurred to me right after #3811 that we actually have cfg_if in our imports, so using it more should provide better ergonomics and prevent issues like #3811 happen again (i.e. "I forgot to add #[cfg()] to some constructs but not others").

I plan to adjust #3803 to use cfg_if!{} in rustup-init.rs as well, where we also tend to use quite a lot of conditionally compiled constructs.

[djc]

A simpler way of doing this would be to just move all the test-gated implementations into an inline module which is guarded with #[cfg(feature = "test")] and a single use statement that exports from the inline module. I think I'd prefer that over the cfg_if!{} approach, which is less standard and noisier.