Rewrite hack/mock-unittest-data/gomod.sh in Python

[eskultety]

Having a structured logic instead of an inline subshell heredoc would enhance readability of the helper script significantly and the potentially larger number of lines as a result of such a rewrite is an acceptable trade-off.

[brunoapimentel]

I have always wondered if automatically genering mock data for a unit test is worth the effort. I'd be happier if we could further break down the massive _resolve_gomod function and rely on a few simpler unit tests, and completely drop down the mock generation part.