[NFC] Convert NameScope from struct to class (#4623)

This is a preparation change for adding name poisoning support
(https://github.com/carbon-language/carbon-lang/issues/4622), which is
expected to require more elaborate logic around NameScope since a name
can be not defined yet, defined, or poisoned.

The API separates looking up a name from getting the full entry since we
have cases where the entries are invalidated between the time we're
looking for the name and when we access (and sometimes modify) the
entry.

This change has the following benefits:
* `names` and `name_map` are internal to `NameScope` and are guaranteed
to match.
* `extended_scopes` and `import_ir_scopes` can not be manipulated (only
new scopes can be added).
* `inst_id`, `name_id` and `parent_scope_id` are constants.
* `has_error` can only be mutated from false to true.

---------

Co-authored-by: jonmeow <jperkins@google.com>

toolchain/check/check.cpp

@@ -198,7 +198,7 @@ static auto ImportCurrentPackage(Context& context, UnitInfo& unit_info,
       unit_info.package_imports[import_map_lookup.value()];
 
   if (self_import.has_load_error) {
-    context.name_scopes().Get(SemIR::NameScopeId::Package).has_error = true;
+    context.name_scopes().Get(SemIR::NameScopeId::Package).set_has_error();
   }
 
   ImportLibrariesFromCurrentPackage(
@@ -208,7 +208,7 @@ static auto ImportCurrentPackage(Context& context, UnitInfo& unit_info,
 
   context.scope_stack().Push(
       package_inst_id, SemIR::NameScopeId::Package, SemIR::SpecificId::Invalid,
-      context.name_scopes().Get(SemIR::NameScopeId::Package).has_error);
+      context.name_scopes().Get(SemIR::NameScopeId::Package).has_error());
 }
 
 // Imports all other packages (excluding the current package).