feat: nested resources, yaml output, deduped collection names (#37)

The AEPs allow deduplicating collection names (for example, 
removing the "book" from "book-editions" in the path). Since
this is a fairly common practice, make it something that aepc
will do for you by default.

Also fixing a few places where multiple nested children was not working,
as well as additional issues that came from a hyphen in the resource singular.

Also started building out an internal/ package - eventually most packages
will move there, as aepc is a first and foremost a command-line interface
and intended to be used as a binary.

A utils library was added to do basic casing, as it is common to 
do so.

Adding yaml support for OAS - this faciliates the aepc
examples usage in the aep.dev site.

Adding some missing parameters in the list API for 
OAS.

cmd/root.go

@@ -74,7 +74,7 @@ func ProcessInput(inputFile, outputFilePrefix string) error {
 		return fmt.Errorf("error parsing service: %w", err)
 	}
 	proto, err := proto.WriteServiceToProto(ps, outputDir)
-	if(err != nil) {
+	if err != nil {
 		return fmt.Errorf("error writing service proto %w", err)
 	}
 	protoFile := fmt.Sprintf("%s.proto", outputFilePrefix)
@@ -93,6 +93,16 @@ func ProcessInput(inputFile, outputFilePrefix string) error {
 		return fmt.Errorf("error writing file: %w", err)
 	}
 	fmt.Printf("output openapi file: %s\n", openapiFile)
+	yamlOpenAPI, err := yaml.JSONToYAML(openapi)
+	if err != nil {
+		return fmt.Errorf("error converting openapi json to yaml: %w", err)
+	}
+	yamlOpenAPIFile := fmt.Sprintf("%s_openapi.yaml", outputFilePrefix)
+	err = WriteFile(yamlOpenAPIFile, yamlOpenAPI)
+	if err != nil {
+		return fmt.Errorf("error writing yaml file: %w", err)
+	}
+	fmt.Printf("output openapi yaml file: %s\n", yamlOpenAPIFile)
 	return nil
 }