chore: tidy up pkgs (#636)

for readability and towards potentially making helmfile usable as a go library

pkg/tmpl/text_renderer.go

@@ -0,0 +1,30 @@
+package tmpl
+
+type templateTextRenderer struct {
+	ReadText func(string) ([]byte, error)
+	Context  *Context
+	Data     interface{}
+}
+
+type TextRenderer interface {
+	RenderTemplateText(text string) (string, error)
+}
+
+func NewTextRenderer(readFile func(filename string) ([]byte, error), basePath string, data interface{}) *templateTextRenderer {
+	return &templateTextRenderer{
+		ReadText: readFile,
+		Context: &Context{
+			basePath: basePath,
+			readFile: readFile,
+		},
+		Data: data,
+	}
+}
+
+func (r *templateTextRenderer) RenderTemplateText(text string) (string, error) {
+	buf, err := r.Context.RenderTemplateToBuffer(text, r.Data)
+	if err != nil {
+		return "", err
+	}
+	return buf.String(), nil
+}