Resolve core types

This adds support for completion and goto definition of
types defined within the "core" crate. The core crate is
added as a dependency to each crate in the project.

The core crate exported it's own prelude. This caused
now all crates to inherit the core crates prelude instead
of the std crates. In order to avoid the problem the
prelude resolution has been changed to overwrite
an already resolved prelude if this was set to a crate
named core - in order to pick a better prelude like std.

Fixes #2199

crates/ra_project_model/src/sysroot.rs

@@ -27,6 +27,10 @@ struct SysrootCrateData {
 }
 
 impl Sysroot {
+    pub fn core(&self) -> Option<SysrootCrate> {
+        self.by_name("core")
+    }
+
     pub fn std(&self) -> Option<SysrootCrate> {
         self.by_name("std")
     }