Parse all kinds of declarations at function scope. (#4779)

These don't fully work in check and beyond yet, because they're not
added into lexical lookup, but already mostly do the right thing.

Per #3407, disallow namespace declarations anywhere other than at file
scope for now.

We don't treat statements starting with a packaging introducer keyword
(`package`, `library`, `import`) as declarations because they're
sufficiently unlikely to occur that the error recovery doesn't seem
important, and this avoids needing to disambiguate `package.` at the
start of an expression.

toolchain/parse/handle_expr.cpp

@@ -156,6 +156,12 @@ auto HandleExprInPostfix(Context& context) -> void {
     }
     case Lex::TokenKind::Package: {
       context.AddLeafNode(NodeKind::PackageExpr, context.Consume());
+      if (context.PositionKind() != Lex::TokenKind::Period) {
+        CARBON_DIAGNOSTIC(ExpectedPeriodAfterPackage, Error,
+                          "expected `.` after `package` expression");
+        context.emitter().Emit(*context.position(), ExpectedPeriodAfterPackage);
+        state.has_error = true;
+      }
       context.PushState(state);
       break;
     }