Support lists of structs containing refs and ClearResolvedReferences (#435)

Implements aws-controllers-k8s/runtime#121

Description of changes:
This PR abstracts out the process of iterating through all references into a separate method called `iterReferenceValues`. `iterReferenceValues` produces Go code that drills down into the spec for every ref, iterating through any slices as necessary. Once it reaches the ref object, it calls a callback which can be used by other methods for logic pertaining to accessing the refs. 

This change allows `ResolveReferencesForField` to access all refs within lists of structs (or even within lists of lists of structs, etc.). `iterReferenceValues` is also used to implement `ClearResolvedReferencesForField`, which simply sets the concrete value to `nil` if it detects a non-nil value in the ref field (or `len > 0` for lists of refs). 

This PR also removes `hasNonNilReferences`. `hasNonNilReferences` was being used to indicate whether there were any references inside the resource, which required another set of iterating through all ref fields. Instead, each `resolveReferencesFor*` returns a boolean which indicates whether it found a reference during its iteration.

Below is a Gist that shows the output of running the code-generator on the current EC2 `generator.yaml`:
https://gist.github.com/RedbackThomson/8e11cbbe96065a4eb812c387665c747d

For testing:
`ec2` and `lambda` already have a series of tests for references. Both of these tests are passing, but they were also passing before this PR. I have a branch of the `ec2-controller` with additional tests using `RouteTables`, which have lists of structs containing refs, and all of those are passing as well. 

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

Author: RedbackThomson

pkg/generate/ack/controller.go

@@ -172,11 +172,8 @@ var (
 		"GoCodeIncompleteLateInitialization": func(r *ackmodel.CRD, resVarName string, indentLevel int) string {
 			return code.IncompleteLateInitialization(r.Config(), r, resVarName, indentLevel)
 		},
-		"GoCodeReferencesValidation": func(r *ackmodel.CRD, sourceVarName string, indentLevel int) string {
-			return code.ReferenceFieldsValidation(r, sourceVarName, indentLevel)
-		},
-		"GoCodeContainsReferences": func(r *ackmodel.CRD, sourceVarName string) string {
-			return code.ReferenceFieldsPresent(r, sourceVarName)
+		"GoCodeReferencesValidation": func(f *ackmodel.Field, sourceVarName string, indentLevel int) string {
+			return code.ReferenceFieldsValidation(f, sourceVarName, indentLevel)
 		},
 		"CheckNilFieldPath": func(f *ackmodel.Field, sourceVarName string) string {
 			return code.CheckNilFieldPath(f, sourceVarName)
@@ -196,6 +193,9 @@ var (
 		"GoCodeResolveReference": func(f *ackmodel.Field, sourceVarName string, indentLevel int) string {
 			return code.ResolveReferencesForField(f, sourceVarName, indentLevel)
 		},
+		"GoCodeClearResolvedReferences": func(f *ackmodel.Field, targetVarName string, indentLevel int) string {
+			return code.ClearResolvedReferencesForField(f, targetVarName, indentLevel)
+		},
 	}
 )
 

pkg/generate/ack/runtime_test.go

@@ -126,8 +126,12 @@ func (frm *fakeRM) LateInitialize(context.Context, acktypes.AWSResource) (acktyp
 	return nil, nil
 }
 
-func (frm *fakeRM) ResolveReferences(context.Context, rtclient.Reader, acktypes.AWSResource) (acktypes.AWSResource, error) {
-	return nil, nil
+func (frm *fakeRM) ResolveReferences(context.Context, rtclient.Reader, acktypes.AWSResource) (acktypes.AWSResource, bool, error) {
+	return nil, false, nil
+}
+
+func (frm *fakeRM) ClearResolvedReferences(acktypes.AWSResource) acktypes.AWSResource {
+	return nil
 }
 
 func (frm *fakeRM) IsSynced(context.Context, acktypes.AWSResource) (bool, error) {