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/sem_ir/file.cpp

@@ -24,7 +24,7 @@ File::File(const Parse::Tree* parse_tree, CheckIRId check_ir_id,
     : parse_tree_(parse_tree),
       check_ir_id_(check_ir_id),
       package_id_(packaging_decl ? packaging_decl->names.package_id
-                                 : IdentifierId::None),
+                                 : PackageNameId::None),
       library_id_(packaging_decl ? LibraryNameId::ForStringLiteralValueId(
                                        packaging_decl->names.library_id)
                                  : LibraryNameId::Default),