Refactor handling of cross-package imports. (#3783)

This revamps the support for cross-package imports, making them look
more like a namespace. The planned model is mentioned on
[#toolchain](https://discord.com/channels/655572317891461132/655578254970716160/1217586076022210670).
This does not implement name lookup into the new namespace structure.

A few key changes in this PR (it's a little sprawling) are:

- Moves logic for adding package imports from context.* to import.*
- Remove SemIR::Import, which was the prior model. This is instead now a
SemIR::Namespace with the NameScope getting a new import_ir_scopes
field.
- Allow SemIR::Namespace to use Parse::ImportDirectiveId in addition to
the prior Parse::NamespaceId
- The import_ir_scopes field includes a NameScopeId so that as we
traverse to child namespaces, we can directly perform name lookup in the
other IR.
- is_closed_import now tracks whether a namespace comes from a different
package. This has a diagnostic implemented in decl_name_stack.

toolchain/parse/handle_import_and_package.cpp

@@ -5,6 +5,7 @@
 #include "toolchain/base/value_store.h"
 #include "toolchain/lex/tokenized_buffer.h"
 #include "toolchain/parse/context.h"
+#include "toolchain/parse/node_ids.h"
 #include "toolchain/parse/node_kind.h"
 
 namespace Carbon::Parse {
@@ -75,7 +76,8 @@ static auto HandleDirectiveContent(Context& context,
                                    NodeKind directive,
                                    llvm::function_ref<void()> on_parse_error)
     -> void {
-  Tree::PackagingNames names{.node = NodeId(state.subtree_start)};
+  Tree::PackagingNames names{
+      .node_id = ImportDirectiveId(NodeId(state.subtree_start))};
   if (directive != NodeKind::LibraryDirective) {
     if (auto package_name_token =
             context.ConsumeIf(Lex::TokenKind::Identifier)) {