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/check/eval.cpp

@@ -506,7 +506,6 @@ auto TryEvalInst(Context& context, SemIR::InstId inst_id, SemIR::Inst inst)
     case SemIR::BranchIf::Kind:
     case SemIR::BranchWithArg::Kind:
     case SemIR::ImplDecl::Kind:
-    case SemIR::Import::Kind:
     case SemIR::Param::Kind:
     case SemIR::ReturnExpr::Kind:
     case SemIR::Return::Kind: