addrs: PartialExpandedResource module address accessors

This is kinda awkward because this address type represents both resources
whose own instances are not expanded yet and resources belonging to whole
modules whose instance keys haven't been expanded yet, and those two cases
have different address types.

However, in return for this awkward API only for the rare case where we
need to isolate the module instance address, the rest of the system gets
to not worry very much about this distinction: it can share most code
between the two cases, since they both need similar evaluation treatment
anyway.

internal/addrs/partial_expanded.go

@@ -323,6 +323,34 @@ func (per PartialExpandedResource) KnownModuleInstancePrefix() ModuleInstance {
 	return per.module.KnownPrefix()
 }
 
+// ModuleInstance returns the fully-qualified [ModuleInstance] that this
+// partial-expanded resource belongs to, but only if its module instance
+// address is fully known.
+//
+// The second return value is false if the module instance address is not
+// fully expanded, in which case the first return value is invalid. Use
+// [PartialExpandedResource.PartialExpandedModule] instead in that case.
+func (per PartialExpandedResource) ModuleInstance() (ModuleInstance, bool) {
+	if len(per.module.unexpandedSuffix) != 0 {
+		return nil, false
+	}
+	return per.module.expandedPrefix, true
+}
+
+// PartialExpandedModule returns a [PartialExpandedModule] address describing
+// the partially-unknown module instance address that the resource belongs to,
+// but only if the module instance address is not fully known.
+//
+// The second return value is false if the module instance address is actually
+// fully expanded, in which case the first return value is invalid. Use
+// [PartialExpandedResource.ModuleInstance] instead in that case.
+func (per PartialExpandedResource) PartialExpandedModule() (PartialExpandedModule, bool) {
+	if len(per.module.unexpandedSuffix) == 0 {
+		return PartialExpandedModule{}, false
+	}
+	return per.module, true
+}
+
 // String returns a string representation of the pattern which uses the special
 // placeholder "[*]" to represent positions where instance keys are not yet
 // known.