I bet it's due go1.15 module-mode be enabled only if you're inside a
go module, and on 1.16 it has changed to be on regardless. see issue
http://golang.org/issues/40276
Perhaps you can get it working on 1.15 with the following: (I didn't
tested, just shooting)
$ cd $(mktemp -d)
$ go mod init tmp
$ GO111MODULE=on go get golang.org/x/tools/...
The temporary module to download is also required on 1.15, but should
not be on tip, I think the issue for that is http://golang.org/issues/40276