chore: tidy up pkgs (#636)

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

pkg/tmpl/context_tmpl.go

@@ -0,0 +1,41 @@
+package tmpl
+
+import (
+	"bytes"
+	"github.com/Masterminds/sprig"
+	"text/template"
+)
+
+func (c *Context) stringTemplate() *template.Template {
+	funcMap := sprig.TxtFuncMap()
+	for name, f := range c.createFuncMap() {
+		funcMap[name] = f
+	}
+	tmpl := template.New("stringTemplate").Funcs(funcMap)
+	if c.preRender {
+		tmpl.Option("missingkey=zero")
+	} else {
+		tmpl.Option("missingkey=error")
+	}
+	return tmpl
+}
+
+func (c *Context) RenderTemplateToBuffer(s string, data ...interface{}) (*bytes.Buffer, error) {
+	var t, parseErr = c.stringTemplate().Parse(s)
+	if parseErr != nil {
+		return nil, parseErr
+	}
+
+	var tplString bytes.Buffer
+	var d interface{}
+	if len(data) > 0 {
+		d = data[0]
+	}
+	var execErr = t.Execute(&tplString, d)
+
+	if execErr != nil {
+		return &tplString, execErr
+	}
+
+	return &tplString, nil
+}