Add more package and import support. (#3402)

This should cover:

```
library "lib" api;
import Foo library default;
import library default;
import library "lib";
```

This splits out `PackageName` and `LibraryName` to their own parse nodes
so that checking can ignore them and still get a balanced parse tree
(otherwise, we essentially need to implement handling of the parse nodes
only to remove the identifiers/string literals -- the optional names
mean we can't blindly do that as before). For reference, these nodes
don't need to be handled because CheckParseTree will need to directly
funnel import information along with checked IRs.

toolchain/parse/handle_decl_scope_loop.cpp

@@ -30,11 +30,15 @@ auto HandleDeclScopeLoop(Context& context) -> void {
       context.PopAndDiscardState();
       return;
     }
-    // `import` and `package` manage their packaging state.
+    // `import`, `library`, and `package` manage their packaging state.
     case Lex::TokenKind::Import: {
       context.PushState(State::Import);
       return;
     }
+    case Lex::TokenKind::Library: {
+      context.PushState(State::Library);
+      return;
+    }
     case Lex::TokenKind::Package: {
       context.PushState(State::Package);
       return;