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/lower/handle.cpp

@@ -217,11 +217,6 @@ auto HandleImplDecl(FunctionContext& /*context*/, SemIR::InstId /*inst_id*/,
   FatalErrorIfEncountered(inst);
 }
 
-auto HandleImport(FunctionContext& /*context*/, SemIR::InstId /*inst_id*/,
-                  SemIR::Import inst) -> void {
-  FatalErrorIfEncountered(inst);
-}
-
 auto HandleImportRefUnused(FunctionContext& /*context*/,
                            SemIR::InstId /*inst_id*/,
                            SemIR::ImportRefUnused inst) -> void {