"any company can hire programmers overseas and just import the source code (git clone) and how is any govt supposed to tax that?"

In South America many countries tax imported professional services (including the outsourcing of source code development) - the way they monitor those types of activities is by tracking all outgoing payments from the country - if you can't pay your service provider, you can't get the service provided.

Therefore the government forces you to report what services you are importing in order to pay for them, and in doing so makes them taxable.