Factor out the sdist git check as a separate script

[harendra-kumar]

• It can be useful to run independently, and it will be modular that way. The main script will be decluttered.
• We can also allow external packcheck scripts to be run as packcheck subcommands if they are in path like git. For example packcheck-sdist-git-check script can be invoked by packcheck sdist-git-check.

[harendra-kumar]

Minimally we should separate out the git check functionality as a separate function.