Fix libcore not being included in rust-lang/rust module tree

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)]`.

crates/cfg/src/lib.rs

@@ -1,4 +1,4 @@
-//! cfg defines conditional compiling options, `cfg` attibute parser and evaluator
+//! cfg defines conditional compiling options, `cfg` attribute parser and evaluator
 
 mod cfg_expr;
 mod dnf;
@@ -59,6 +59,20 @@ pub struct CfgDiff {
 }
 
 impl CfgDiff {
+    /// Create a new CfgDiff. Will return None if the same item appears more than once in the set
+    /// of both.
+    pub fn new(enable: Vec<CfgAtom>, disable: Vec<CfgAtom>) -> Option<CfgDiff> {
+        let mut occupied = FxHashSet::default();
+        for item in enable.iter().chain(disable.iter()) {
+            if !occupied.insert(item) {
+                // was present
+                return None;
+            }
+        }
+
+        Some(CfgDiff { enable, disable })
+    }
+
     /// Returns the total number of atoms changed by this diff.
     pub fn len(&self) -> usize {
         self.enable.len() + self.disable.len()