The completion of cfg will look at the enabled cfg keys when
performing completion.
It will also look crate features when completing a feature cfg
option. A fixed list of known values for some cfg options are
provided.
For unknown keys it will look at the enabled values for that cfg key,
which means that completion will only show enabled options for those.
If you are opening libcore from rust-lang/rust as opposed to e.g.
goto definition from some other crate which would use the sysroot
instance of libcore, a `#![cfg(not(test))]` would previously have made
all the code excluded from the module tree, breaking the editor
experience.
This puts in a slight hack that checks for the crate name "core" and
turns off `#[cfg(test)]`.
