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/sem_ir/file.cpp

@@ -234,7 +234,6 @@ static auto GetTypePrecedence(InstKind kind) -> int {
     case FieldDecl::Kind:
     case FunctionDecl::Kind:
     case ImplDecl::Kind:
-    case Import::Kind:
     case ImportRefUnused::Kind:
     case InitializeFrom::Kind:
     case InterfaceDecl::Kind:
@@ -483,7 +482,6 @@ static auto StringifyTypeExprImpl(const SemIR::File& outer_sem_ir,
       case FieldDecl::Kind:
       case FunctionDecl::Kind:
       case ImplDecl::Kind:
-      case Import::Kind:
       case ImportRefUnused::Kind:
       case InitializeFrom::Kind:
       case InterfaceDecl::Kind:
@@ -549,7 +547,6 @@ auto GetExprCategory(const File& file, InstId inst_id) -> ExprCategory {
       case FieldDecl::Kind:
       case FunctionDecl::Kind:
       case ImplDecl::Kind:
-      case Import::Kind:
       case ImportRefUnused::Kind:
       case Namespace::Kind:
       case Return::Kind: