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.

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

toolchain/parse/node_ids.h

@@ -81,6 +81,7 @@ using AnyStatementId =
 using AnyRequirementId = NodeIdInCategory<NodeCategory::Requirement>;
 using AnyNonExprIdentifierNameId =
     NodeIdInCategory<NodeCategory::NonExprIdentifierName>;
+using AnyPackageNameId = NodeIdInCategory<NodeCategory::PackageName>;
 
 // NodeId with kind that matches one of the `T::Kind`s.
 template <typename... T>