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/lex/token_kind_test.cpp

@@ -21,7 +21,7 @@ constexpr llvm::StringLiteral SymbolRegex =
 
 // We restrict keywords to be lowercase ASCII letters and underscores with a few
 // specific exceptions.
-constexpr llvm::StringLiteral KeywordRegex = "[a-z_]+|Self|String";
+constexpr llvm::StringLiteral KeywordRegex = "[a-z_]+|Core|Self|String";
 
 #define CARBON_TOKEN(TokenName)                           \
   TEST(TokenKindTest, TokenName) {                        \