resolver: fix case-insensitive file name collision

for compatibility with Windows/Mac OS X, Go does not allow files with
identical case-insensitive names.
this commit changes the key of the 'files' map to use the lower case
file name, keeping the original file name as a property on the File
object.

plugin/resolvergen/resolver.go

@@ -110,9 +110,12 @@ func (m *Plugin) generatePerSchema(data *codegen.Data) error {
 
 	for _, o := range objects {
 		if o.HasResolvers() {
-			fn := gqlToResolverName(data.Config.Resolver.Dir(), o.Position.Src.Name, data.Config.Resolver.FilenameTemplate)
+			fnCase := gqlToResolverName(data.Config.Resolver.Dir(), o.Position.Src.Name, data.Config.Resolver.FilenameTemplate)
+			fn := strings.ToLower(fnCase)
 			if files[fn] == nil {
-				files[fn] = &File{}
+				files[fn] = &File{
+					name: fnCase,
+				}
 			}
 
 			caser := cases.Title(language.English, cases.NoLower)
@@ -150,25 +153,28 @@ func (m *Plugin) generatePerSchema(data *codegen.Data) error {
 			}
 
 			resolver := Resolver{o, f, rewriter.GetPrevDecl(structName, f.GoFieldName), comment, implementation}
-			fn := gqlToResolverName(data.Config.Resolver.Dir(), f.Position.Src.Name, data.Config.Resolver.FilenameTemplate)
+			fnCase := gqlToResolverName(data.Config.Resolver.Dir(), f.Position.Src.Name, data.Config.Resolver.FilenameTemplate)
+			fn := strings.ToLower(fnCase)
 			if files[fn] == nil {
-				files[fn] = &File{}
+				files[fn] = &File{
+					name: fnCase,
+				}
 			}
 
 			files[fn].Resolvers = append(files[fn].Resolvers, &resolver)
 		}
 	}
 
-	for filename, file := range files {
-		file.imports = rewriter.ExistingImports(filename)
-		file.RemainingSource = rewriter.RemainingSource(filename)
+	for _, file := range files {
+		file.imports = rewriter.ExistingImports(file.name)
+		file.RemainingSource = rewriter.RemainingSource(file.name)
 	}
 	newResolverTemplate := resolverTemplate
 	if data.Config.Resolver.ResolverTemplate != "" {
 		newResolverTemplate = readResolverTemplate(data.Config.Resolver.ResolverTemplate)
 	}
 
-	for filename, file := range files {
+	for _, file := range files {
 		resolverBuild := &ResolverBuild{
 			File:                file,
 			PackageName:         data.Config.Resolver.Package,
@@ -192,7 +198,7 @@ func (m *Plugin) generatePerSchema(data *codegen.Data) error {
 		err := templates.Render(templates.Options{
 			PackageName: data.Config.Resolver.Package,
 			FileNotice:  fileNotice.String(),
-			Filename:    filename,
+			Filename:    file.name,
 			Data:        resolverBuild,
 			Packages:    data.Config.Packages,
 			Template:    newResolverTemplate,
@@ -230,6 +236,7 @@ type ResolverBuild struct {
 }
 
 type File struct {
+	name string
 	// These are separated because the type definition of the resolver object may live in a different file from the
 	// resolver method implementations, for example when extending a type in a different graphql schema file
 	Objects         []*codegen.Object