instances: "skipUnregistered" instead of "skipUnknown" internally

Recently we've introduced the idea of the expansion of a particular module
or resource not yet being known, which has inadvertently introduced some
ambiguity about what "unknown" means in this package: "unknown" previously
represented "not registered yet", which is different than having
explicitly registered that we don't know.

To remove that ambiguity, we'll now use the term "unregistered" to talk
about the "not registered yet" situation, and reserve "unknown" for when
we know that we definitely don't know.

This is only an internal naming change within the package and doesn't
affect any exported API.

internal/instances/expander.go

@@ -135,10 +135,10 @@ func (e *Expander) ExpandModule(addr addrs.Module) []addrs.ModuleInstance {
 	return e.expandModule(addr, false)
 }
 
-// expandModule allows skipping unexpanded module addresses by setting skipUnknown to true.
+// expandModule allows skipping unexpanded module addresses by setting skipUnregistered to true.
 // This is used by instances.Set, which is only concerned with the expanded
 // instances, and should not panic when looking up unknown addresses.
-func (e *Expander) expandModule(addr addrs.Module, skipUnknown bool) []addrs.ModuleInstance {
+func (e *Expander) expandModule(addr addrs.Module, skipUnregistered bool) []addrs.ModuleInstance {
 	if len(addr) == 0 {
 		// Root module is always a singleton.
 		return singletonRootModule
@@ -153,7 +153,7 @@ func (e *Expander) expandModule(addr addrs.Module, skipUnknown bool) []addrs.Mod
 	// (moduleInstances does plenty of allocations itself, so the benefit of
 	// pre-allocating this is marginal but it's not hard to do.)
 	parentAddr := make(addrs.ModuleInstance, 0, 4)
-	ret := e.exps.moduleInstances(addr, parentAddr, skipUnknown)
+	ret := e.exps.moduleInstances(addr, parentAddr, skipUnregistered)
 	sort.SliceStable(ret, func(i, j int) bool {
 		return ret[i].Less(ret[j])
 	})
@@ -521,13 +521,14 @@ func newExpanderModule() *expanderModule {
 var singletonRootModule = []addrs.ModuleInstance{addrs.RootModuleInstance}
 
 // if moduleInstances is being used to lookup known instances after all
-// expansions have been done, set skipUnknown to true which allows addrs which
-// may not have been seen to return with no instances rather than panicking.
-func (m *expanderModule) moduleInstances(addr addrs.Module, parentAddr addrs.ModuleInstance, skipUnknown bool) []addrs.ModuleInstance {
+// expansions have been done, set skipUnregistered to true which allows addrs
+// which may not have been seen to return with no instances rather than
+// panicking.
+func (m *expanderModule) moduleInstances(addr addrs.Module, parentAddr addrs.ModuleInstance, skipUnregistered bool) []addrs.ModuleInstance {
 	callName := addr[0]
 	exp, ok := m.moduleCalls[addrs.ModuleCall{Name: callName}]
 	if !ok {
-		if skipUnknown {
+		if skipUnregistered {
 			return nil
 		}
 		// This is a bug in the caller, because it should always register
@@ -550,7 +551,7 @@ func (m *expanderModule) moduleInstances(addr addrs.Module, parentAddr addrs.Mod
 				continue
 			}
 			instAddr := append(parentAddr, step)
-			ret = append(ret, inst.moduleInstances(addr[1:], instAddr, skipUnknown)...)
+			ret = append(ret, inst.moduleInstances(addr[1:], instAddr, skipUnregistered)...)
 		}
 		return ret
 	}