Implement #4864: Core is a keyword (#4909)

Change representation of package names from `IdentifierId` to
`PackageNameId`, and add a special value `PackageNameId::Core` for the
Core package. Add a `Core` expression to name the Core package, and
support for parsing the `Core` keyword in `package` and `import`
declarations.

For now, I've made no changes to instruction fingerprinting or name
mangling. This means that fingerprints and mangled names will collide
between names in the `Core` package and names in a `r#Core` package. See
#4908.

---------

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>

toolchain/check/handle_name.cpp

@@ -244,4 +244,12 @@ auto HandleParseNode(Context& context, Parse::PackageExprId node_id) -> bool {
   return true;
 }
 
+auto HandleParseNode(Context& context, Parse::CoreNameExprId node_id) -> bool {
+  // TODO: Unqualified lookup will never find anything; perform lookup directly
+  // into file scope.
+  context.node_stack().Push(
+      node_id, HandleNameAsExpr(context, node_id, SemIR::NameId::Core));
+  return true;
+}
+
 }  // namespace Carbon::Check