Extract a method from `vendored_license` for better usage in prepend_mod

What does this MR do and why?

This MR extracts a method from vendored_license method named available_license.
It can create some convenience for better prepend_mod.

Example: MR in Jihulab Making Mulan-PSL License appears in License Template drop-down option list

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

/cc @songhuangcn @daveliu