cmd/go: update "GOPATH environment variable" section to be module-mode first

[hyangah]

Current version of GOPATH_environment_variable (as of go1.20rc1)
is still focusing more on the use/layout of GOPATH in GOPATH-mode and is a common source of confusion for users.

It is followed by a tiny section GOPATH_and_Modules but this is really easy to miss.

cc @bcmills @matloob @rsc

[gopherbot]

Change https://go.dev/cl/475640 mentions this issue: cmd/go: move module info up in GOPATH help