Refactor the dependencies for spec-gen-sdk

[raych1]

Here are the goals:

• reduce the unnecessary dependencies
• replace the dependencies which haven't been GAed or are inactively maintained

These are the candidates:

• commonmark
• difflib

[JackTn]

For commonmark, necessary to write manully

[JackTn]

For difflib, use another popular repo e.g. diff to replace it