Import paths are used to figure out where to fetch the source code from. To simplify a bit, if you see `example.com/foo/bar`, it works because there's an appropriate meta tag at https://example.com/foo/bar?go-get=1 . The import paths are pretty directly used as URLs.
Ah, TIL, thanks!
I meant more "why must the import syntax use specifically this scheme to denote versions". I guess backwards compat?