People are (understandably) confused by creating go.mod files in GOPATH/src
and then having the go command not use modules in those directories.
We can't change that behavior (or we'll break non-module users of GOPATH)
but we can force 'go mod' (including 'go mod -init') to fail loudly in that case.

If this is not enough, the next step would be to print a warning every time
the go command is run in a GOPATH/src directory with a go.mod but
module mode hasn't triggered. But that will annoy all the non-module users.
Hopefully anyone confused will eventually run a 'go mod' command of
some kind, which will fail loudly.

Fixes #26365.

Change-Id: I8c5fe987fbc3f8d2eceb1138e6862a391ade150c
Reviewed-on: https://go-review.googlesource.com/124708Reviewed-by: Bryan C. Mills <bcmills@google.com>