Add imports of enclosing scopes. (#3575)

Namespaces are copied, which means also adding their name to the
underlying instruction. It happened not to be done previously; the name
was only in name lookup.

Since the only import supported right now is the default import,
functionality is limited; in the future I'll need to deal with namespace
vs package conflicts.

Tests of namespace imports are under "namespace" -- I figured this would
be best for scaling as more instructions get support.

This also improves some debugging-related output that I was trying to
use while trying to build the support.

---------

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

toolchain/check/context.cpp

@@ -181,6 +181,7 @@ auto Context::ResolveIfLazyImportRef(SemIR::InstId inst_id) -> void {
       // TODO: Fill this in better.
       auto function_id =
           functions().Add({.name_id = SemIR::NameId::Invalid,
+                           .enclosing_scope_id = SemIR::NameScopeId::Invalid,
                            .decl_id = inst_id,
                            .implicit_param_refs_id = SemIR::InstBlockId::Empty,
                            .param_refs_id = SemIR::InstBlockId::Empty,