Extend go.mk to be able to download from different additional sources

Hi,

I have the use-case from some ports I maintain which are related or are 
based on go.

Upstream does not provide the go.mod via goproxy but e.g. only on gitlab 
or github.
Therefor the go modules approve to build the port is not possible.

The extension enables the go.mk to more flexibly download the go.mod 
from different sources.

Would be nice if some can have a look to the review:
https://reviews.freebsd.org/D45631

and share your opinion.
The modification is designed to have no effect to existing ports/packages.

Thanks a lot!

Gruß
Matthias

-- 

"Programming today is a race between software engineers striving to
build bigger and better idiot-proof programs, and the universe trying to
produce bigger and better idiots. So far, the universe is winning." --
Rich Cook