cmd/go: remove test dependency on gopkg.in service #54503
Description
#!watchflakes
post <- pkg == "cmd/go" && test == "TestScript" && (`reading https://gopkg\.in` || `https fetch: Get "https://gopkg\.in`)
The gopkg.in service has occasional downtime (most recently niemeyer/gopkg#78).
Some of the cmd/go tests of gopkg.in modules are isolated from outages by the Go module proxy, but unfortunately others fail when the service is down.
As far as I can tell, the go-import metadata served by gopkg.in is mostly static. (It may add some value in serving up-to-date go-source metadata, but cmd/go does not depend on that metadata.) Since we're going to have a test-local HTTPS server anyway for #27494, perhaps we can migrate the gopkg.in tests to use that server with static go-import metadata instead.