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/import.cpp

@@ -450,13 +450,13 @@ auto ImportLibrariesFromCurrentPackage(
 auto ImportLibrariesFromOtherPackage(Context& context,
                                      SemIR::TypeId namespace_type_id,
                                      SemIR::InstId import_decl_id,
-                                     IdentifierId package_id,
+                                     PackageNameId package_id,
                                      llvm::ArrayRef<SemIR::ImportIR> import_irs,
                                      bool has_load_error) -> void {
   CARBON_CHECK(has_load_error || !import_irs.empty(),
                "There should be either a load error or at least one IR.");
 
-  auto name_id = SemIR::NameId::ForIdentifier(package_id);
+  auto name_id = SemIR::NameId::ForPackageName(package_id);
 
   NamespaceResult result = AddNamespace(
       context, namespace_type_id, name_id, SemIR::NameScopeId::Package,