Merge pull request #55948 from munnerz/patch-2

Automatic merge from submit-queue (batch tested with PRs 55841, 55948, 55945). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

conversion-gen: check for nil pkg in getManualConversionFunctions

**What this PR does / why we need it**:

Following kubernetes/kubernetes#54394, this PR performs a check in getManualConversionFunctions for packages that are not found and prints a warning instead of panicking.

This should improve the UX of conversion-gen, and is also a narrower change than that proposed in #54394, so hopefully can be cherry picked into the release-1.8 etc. branches.

This would allow users to use conversion-gen via the generate-internal-groups.sh script without having to make a second call that 'blanks out' `--extra-peer-dirs`.

/cc @sttts

```release-note
NONE
```

Kubernetes-commit: a83f78efc9f98b71b4604acf6ba14a4a51eceac8

cmd/conversion-gen/generators/conversion.go

@@ -145,6 +145,10 @@ type conversionFuncMap map[conversionPair]*types.Type
 
 // Returns all manually-defined conversion functions in the package.
 func getManualConversionFunctions(context *generator.Context, pkg *types.Package, manualMap conversionFuncMap) {
+	if pkg == nil {
+		glog.Warningf("Skipping nil package passed to getManualConversionFunctions")
+		return
+	}
 	glog.V(5).Infof("Scanning for conversion functions in %v", pkg.Name)
 
 	scopeName := types.Ref(conversionPackagePath, "Scope").Name